BORN: EDUCATION: BIOGRAPHICAL INFORMATION Max R. Zelle’ Polk County, fowa, October ll, 1915 Diploma, Alleman High School, Alleman, Lowa 1932, Valedictorian. B.S. degree in Animal Husbandry, Iowa State College, March, 1937. Ph.D. degree in Genetics, lowa State College, December, 1940. POSITIONS AND EXPERIENCE: 1935-36 1937-38. 1938-40 1940-44 1944-46 1946-48 1948-58 1950-51 1951-54 1947-49 & * 1952-57 1952, '54 and ‘56. 1957-58 1958-60 1960-61 1961-62 1962-69 Asst. County Agricultural Agent, Iowa State Extension Service, Audubon, Iowa ok _ Fellowship in Genetics, Iowa State College Graduate Assistant In Genetics, Iowa State College Assistant Professor of Animal Husbandry (Genetics), Purdue University Ensign and Lt. (4. g-) USNR, Classified Research on Biological Warfare, Camp Detrick, Frederick, Maryland. Biologist, Laboratory of Physical Biology, National Institutes of Health, Bethesda, Maryland Professor of Bacteriology, Cornell University, Ithaca, New York Geneticist, Biology Branch, Division of Biology and Medicine, Atomic Energy Commission, Washington, D. C. (On leave from Cornell Univ.) Consultant, Division of Biology and Medicine, Atomic Energy Commission Consultant, Biology Division, Oak Ridge National Laboratory, ¢ Oak Ridge, Tennessee Summer Guest Investigator, Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee Geneticist, Biology. Branch, Division of Biology and Medicine, Atomic Energy Commission, Washington, D. C. (On leave from Cornell Univ.) Chief, Biology Branch, Division of Biology and Medicine., Atomic Energy Commission, Washington, D. C. Assistant Director for Biological Sciences, Division of Biology and Medicine, Atomic Energy Commission, Washington, D. C. Professor of Genetics and Director, Center for Radiological Sciences, University of Washington, Seattle, Washington Director, Division of Biological and Medical Research, Argonne National Laboratory, Argonne, Illinois Zelle, M. R. -2- POSITIONS AND EXPERIENCE: (Con't) 1958-65 1963-69 1963-72 1965-74 ~ 1969-73 1969 + 1970 1970-73 1970-73 1971-74 1971-75 1971-73 1972 + 1972 1974 ‘1975 + 1974-76 1976-77 1976-77 1977. 1977 1978 1978 1979 1979+ 1980+ Advisor to United States delegation to the United Nations Scientific Committee on the Effects on Atomic Radiation Lecturer in Zoology, University of Chicago, Chicago, Illinois Member of Editorial Board, Mutation Research Co-editor, Advances in Radiation Biology series: Academic Press Member, Radiological Health Study Section, Bureau of Radiological Health, Food and Drug Administration, Department of Health, Education and Welfare Chairman, Department of Radiology and Radiation Biology, Colorado State University, Fort Collins , Colorado Member, Search Committee for Dean of College of Veterinary Medicine and Biomedical Sciences Member, Editorial Board, Applied Microbiology Member, Steering Committee, Western Interstate Nuclear Board, Denver, Colorado Faculty Council, CSU, Representative at large Member, Cultural Programs Advisory Committee, CSU Member of Steering Committee, Faculty Council, CSU Radiation Control Officer and Member, Radiation Safety Committee, CSU Consultant, U. S. Atomic Energy Commission, Plowshare Programs Office -Chairman, Search Committee for Associate Professor of Radiology Member, Intersociety Liason Committee, Health Physics Society Member, Faculty Benefits Committee, CSU Chairman, Faculty Benefits Committee, CSU Chairman, Search Committee for Associate Professor of Radiological Physics Chairman, Search Committee for Director of Collaborative Radiological Health Laboratory Chairman, Search Committee for Head of Microbiology Chairman, Search Committee for Associate Dean, College of Veterinary Medicine and Biomedical Sciences , Consultant, University of Tennessee Institute of Radiation Biology Consultant,American Pharmaceutic Association - Member,CSU Committee on Responsibilities and Standing of the Academic Faculty Member. CSU Advisory Committee for Adult Students. Zelle, M. R. / . -3- HONORS AND PROFESSIONAL SOCIETIES: Honor Student, College of Agriculture, Iowa State University Scabbard and Blade, Military Honorary Alpha Zeta, Agricultural Honorary Sigma Xi Phi Kappa Phi ‘Who's Who in the World (4th Edition, 1978). Who's Who in America Who's Who in the West ‘Who's Who in Atoms Listed in the Blue Book; Leaders of the English Speaking World Dictionary of International Biography Honorary Editorial Board, Photochemistry and Photobiology 1962- -1970 Cosmos Club, Washington, D.C. - American Association for Advancement of Science, Fellow 1955 American Institute of Biological Sciences - American Society for Microbiology American Society of Naturalists American Society for Photobiology Genetics Society of America Radiation Research Society: Councilor in Biology, 1960-63; Secretary-Treasurer Elect 1971; Secretary-Treasurer 1972-75; Member, Committee for Advancement of Radiation Research, 1974-76; Chairman, Education Committee 1974-77 Health Physics Society: President, Central Rocky Mountain Chapter 1976-1977 RESEARCH INTERESTS: Radiation Biology, Genetics, Microbiology Zelle, M. R. a4 BIBLIOGRAPHY Open Literature Publications: . Zelle, M. R. 1942. Genetic constitutions of host and pathogen in mouse typhoid. J. Infectious Diseases 71: 131-152. Tyrrell, W. P., F. N. Andrews, and M. R. Zelle. 1942. Effect of post-pubertal castration on thermo-regulatory function of rat scrotum. Endocrinology 31: 379-383. Shrewsbury, C. L., C. Harper, F. N. Andrews, and M. R. Zelle. 1942. The limitations of oat straw as a roughage for maintenance, lactation and growth in sheep. J. Animal Sci. 1: 126-130. Shrewsbury, C. L., F. N. Andrews, C. Harper, and M. R. Zelle. 1943. The value of alfalfa and certain of its fractions in the nutrition of breeding ewes. J. Animal Sci. 2: 209-220. Gowen, J. W., and M. R. Zelle. 1945. Irradiation effects on genetic resistance of mice to mouse typhoid. J. Infectious Diseases 77: 85-91. Young, G. A., Jr., M. R. Zelle, and R. E. Lincoln. 1946. Respiratory pathogenicity of Bacillus anthracis spores. I. Methods of study and observations on pathogenesis. J. Infectious Diseases 79: 233-246. Zelle, M. R., R. E. Lincoln, and G. A. Young, Jr. 1946. Respiratory pathogenicity of Bacillus anthracis spores. II. Genetic variation in respiratory pathogenicity and invasiveness of colonial variants of B. anthracis. J. Infectious Diseases 79: 247-253. Lincoln, R. E., M. R. Zelle, C. I. Randles, J. L. Roberts, and G. A. Young, Jr. 1946. Respiratory pathogenicity of Bacillus anthracis spores. III. Changes in pathogenicity due to nutritional modifications. J. Infectious Diseases 79: 254—265. Young, G. A., Jr.; and M. R. Zelle. 1946. Respiratory pathogenicity of Bacillus anthracis spores. IV. Chemical-biological synergisms. J. Infectious Diseases 79: 266-271. Jarrett, E. T., Lt. (HC) USN, M. R. Zelle, and A. Hollaender. 1948. Studies of the control of acute respiratory disease among naval recruits. Il. Limitations of ultraviolet irradiation in reducing air-borne bacteria in barracks with low ceilings. Am. J. Hyg. 48: 233-239. Buck, J. Bo, M. L. Keister, and M. R. Zelle. 1950. An areal method for calibrating microburettes. Anal. Chim. Acta 4: 130-134. Zelle, M. R. 1951. A simple single-cell technique for genetic studies of bacteria. J. Bacteriol. 61: 345-349. Zelle, M. R. -5- Open Literature Publications, con't: Zelle, M. R., and J. Lederberg. 1951. Single-cell isolations of diploid heterozygous Escherichia coli. J. Bacteriol. 61: 351-355. Dondero, N. C., and M. R. Zelle. 1953. Observations on the formation and behavior of "conjugation" cells and large bodies in Azotobacter agile. Science 118: 34-36. Dondero, N. C., and M. R. Zelle. 1953. A method for retaining bacterial ’ cells in place for staining and cytochemical tests for light or electron microscopy. J. Histochem. Cytochem. 1: 415-419. Zelle, M. R., and A. Hollaender. 1954. Monochromatic ultraviolet action spectra and quantum yields for inactivation of Tl and T2 Escherichia coli bacteriophages. J. Bacteriol. 68: 210-215. Dondero, N. C., H. I.-Adler, and M. R. Zelle. 1954. Quantification of the Feulgen reaction in bacteria. J. Bacteriol. 68: 483-492. Zelle, M. R., and A. Hollaender. 1955. Effects of radiation on bacteria. In: Radiation Biology, Vol. II: Ultraviolet and Related Radiations. Ed.: A. Hollaender. New York: McGraw-Hill Book Co., Inc., pp; 365-430. Zelle, M. R. 1955. Symposium on radiation effects on cells and bacteria. Part 3. Radiation induced mutations and their implications on the mechanisms of radiation effects on bacteria. Bacteriol. Rev. 19: Hollaender, A., and M. R. Zelle. 1954. The use of action spectra for the evaluation of some basic biological problems. In: Proceedings of the First International Photobiology Congress (Amsterdam, Aug. 23-28, 1954). The Netherlands: H. Veenman and Zonen - Washington, pp. 128-134. Zelle, M. R. 1955. Genetics of microorganisms. In: Annual Review of Microbiology, Vol. 9. Ed.: C. E. Clifton. Stanford, Calif.: Annual Reviews, Inc., pp. 45-96. : Ogg, J. E., H. I. Adler, and M. R. Zelle. 1956. Protection of Escherichia coli against ultraviolet irradiation by catalase and related enzymes. J. Bacteriol. 72: 494-496. Adler, H. I., and M. R. Zelle. 1957. Quantitative evaluation of the effects of various methods of fixation on the appearance of stained structures in the bacterial cell. J. Bacteriol. 73: 526=529. — Ogg, J. E., and M. R. Zelle. 1957. Isolation and characterization of a large cell possibly polyploid strain of Escherichia coli. J. Bacteriol. 74: 477-484, . -. Zelle, M. R. ~6- Open Literature Publications, con't: Zelle, M. R., and J. E. Ogg. 1957. Radiation resistance and genetic segregation in a large cell possibly polyploid strain of Escherichia coli. J. Bacteriol. 74: 484-493. Zelle, M. R., J. E. Ogg, and A. Hollaender. 1957. Differential photo- reactivation of Escherichia coli after exposure to 2650 and 2250A ultraviolet. Poc. Soc. Exptl. Biol. Med. 96: 285-287. Zelle, M. R. 1957. Some observations bearing on the nature of lethal radiation damage in Escherichia coli. In: Proceedings of the Second International Photobiology Congress (Turin, Italy, June 2-8, 1957). Edizioni Minerva Medica, Turin, Italy, pp. 155-166. Zelle, Ms R., J. E. Ogg, and A. Hollaender. 1958. Photoreactivation of — induced mutation and inactivation of Escherichia coli exposed to various wave lengths of monochromatic ultraviolet radiation. J. Bacteriol. 75: 190-198. Zelle, M. R. 1958. Diagnostic X-rays from the viewpoint of a geneticist. Med. Ann. of the District of Columbia, 27: 455-459. Totter, J. R., M. R. Zelle, and H. Hollister. 1958. The biological hazari to man of carbon-14 from nuclear weapons. 19 pp. Washington Technical Report, WASH-1008.. Atomic Energy Commission. Technical Information Service Extension, Oak Ridge, Tennessee. Also published in Science 128: 1490-1495 (1958). Zelle, M. R. 1960. Radioisotopes and the genetic mechanism: Mutagenic aspects. In: Radioisotopes in the Biosphere. Ed.: R. S. Caldecott and L. A. Snyder. Minneapolis, Minn.: Center for Continuation Study of the General Extension Division, University of Minnesota, pp. 160-180. Zelle, M. R. 1960. Modification of radiation effects in bacteria. In: - Developments in Industrial Microbiology. New York: Plenum Press, Vol. 1, Pp- 222-230. e Zelle, M. R. 1960. Biological effects of ultraviolet radiation. IRE Transactions on Medical Electronics, Vol. ME-7, No. 3, pp. 130-135. Zelle, M. R. 1961. Reports on biological effects of radiation-—Somatic and genetic studies: Large-scale programmes. Can. J. Genet. Cytol. 3: 80-81. Zelle, M. R. 1962. Biological and agricultural uses of radioisotopes and radiation. Intern. J. Applied Radiation and Isotopes 13: 319-323. Pergamon Press. Ltd. Zelle, M. R. -j- Open Literature Publications, con't: Zelle, M. R. 1967. Radiation in relation to basic biology. Proceedings of the Conference on Radiation Biology, Oak Ridge, Tenn., Aug. 2-5, 1965, pp. 1-8. U. S. Atomic Energy Commission Division of Technical Information Conf-650747, Sept., 1967. | Krisch, R. E., and M. R. Zelle. 1969. Biological effects of radioactive decay: The role of the transmitation effect. In: Advances in Radiation Biology. New York: Academic Press Inc., Vol. III, pp. 177-213. Fritz, T. E., R. C. Zeman, and M. R. Zelle. 1970. Pathology and familial incidence of thyroiditis in a closed beagle colony. Experimental and Molecular Pathology, 12: 14-30. . Zelle, M. R. 1970. Dr. John W. Gowen, Obituary. Rad. Res. 42: 425-427. Evetkas, Marilyn J., R. E. Krisch, and M. R. Zelle. 1970. Genetic and physiological studies of a large-cell, radiation-resistant strain of Escherichia coli. I. Genetic analysis of Escherichia coli P6. Zelle, M. R. 1971. Dr. Douglas E. Smith - Physiologist and Photobiologist. In: NATO International Advanced Study Institute on Biological Effects of Visible Light. Photochemistry and Photobiology of Photodynamic Action. Research Progress in Organic, Biological and Medicinal , Chemistry. North Holland Publishing Co. Vol. 3, Part 1: 270-273. Zelle, M. R. 1972. Introductory remarks. In: NATO International Advanced Study Institute on Biological Effects of Visible Light. Photochemistry and Photobiology of Photodynamic Action. Research Progress in Organic, Biological and Medicinal Chemistry. North Holland Publishing Co. Vol. 3, Part I: 273-279. , Zelle, M. R. 1972. .Genetic effects of photodynamic action. In: NATO Inter- - national Advanced Study Institute on Biological Effects of Visible Light. Photochemistry and Photobiology of Photodynamic Action. Research Progress in Organic, Biological and Medicinal Chemistry. North Holland Publishing Co. Vol. 3, Part 1: 280-296. Kvetkas, Marilyn J., Robert E. Krisch, and M. R. Zelle. 1971. Phenotypic properties of a large-cell, radiation~resistant strain of Escherichia coli: Effect of variations in the growth rate. Canadian J. Bacteriol., 18:1417-1425. Cleaver, J. E., Kainer, R. A., and M. R. Zelle. 1972. “Cancer eye in Hereford cattle. Radiation repair processes and a comparison of cultured cells | with Xeroderma Pigmentosum in man. 1971. American Journal of Veterinary Research, 33: 1131-1136. Zelle, M. R. 1973. Book Review. "Biomedical Implications of Radiostrontium Exposure", M. Goldman and L. K. Bustad, editors. U. S. Atomic Energy Commission, Office of Information Services. Laboratory Animal Science. In press. Zelle, M. R. | -8- Open Literature Publications, con't: Zelle, M. R. 1974. Human Genetic Concerns. In: Environment and Colorado - A Handbook. Phillip 0. Foss Editor. Published by the Environmental Resources Center, Colorado State University, Fort Collins, Colo. pp. 65-69. Zelle, M. R. Abstracts: | Zelle, M. R., and J. W. Gowen. 1940. On the origin of epidemic virulence. Genetics 25: 140-141. a Zelle, M. R. 1941. On. the mechanism of variegation | in bacterial colonies. Genetics 26: 174. Zelle, M. R., and J. W. Gowen. 1941. Interaction of the genetic constitutions of host and pathogen in mouse typhoid. Genetics 26: 174-175. Zelle, M. R., and J. E. Ogg. 1954. setepee to produce polyploidy in Escherichia coli by camphor vapors. ‘Bacteriol. Proc. 54: 39-40. Ogg, J. E., and M. R. Zelle. 1955. Radiation inactivation kinetics of a presumed polyploid strain of Escherichia coli. Bacteriol. Proc. 55: 58. Zelle, M. R., and R. R. Alexander. 1957. Ultraviolet absorption coeffi- cients and total cell counts of Escherichia coli strains. Bacteriol. Proc. 57: 48. Zelle, M. R. 1958. Studies of a large cell, radiation resistant, possibly polyploid strain derived from Escherichia coli B/r. In: Proceedings of the Tenth International Congress of Gel of Genetics (Montreal, Canada, Aug. 20-27). Toronto, Ontario, Canada: Univ. of Toronto Press, ‘pp. 331-332. , Zelle, M. R. 1959. Biological effects of ultraviolet radiation. In: Digest of Technical Papers, 12th Annual Conference on Electrical Techniques in Medicine and Biology (Philadelphia, Pars Nov. 10-12) New York: Lewis Winner, p. 18. ‘Kvetkas, Marilyn J., and M. R. Zelle. 1967. Genetic analysis of a large- cell, radiation-resistant strain of Escherichia coli. Microbial Genetics Bulletin, Oak Ridge National Laboratory 27: 7-8. Kvetkas, Marilyn J., and Max R. Zelle. 1968. Genetic analysis of a large-cell, radiation-resistant strain of Escherichia coli. Bacteriol. Proc. 68: 62. Kvetkas, Marilyn J., Robert E. Krisch, and Max R. Zelle. 1970. Physiological ‘studies of a large-cell, radiation-resistant, mutant strain of . Escherichia coli. Bacteriol. Proc. 70: 38. Zelle, M. R. 1969. Dr. Douglas E. Smith - Physiologist and photobiologist. In: NATO International Advanced Study Institute on Biological Effects of Visible Light. Photochemistry and Photobiology of "Photodynamic Action." Univ. of Sassari, Sardinia, Italy, Sept. 3-17, 1969, pp. 106-107. Zelle, M. R. -10- Abstracts, con't: Zelle, M. R. 1969. Genetic effects in microorganisms. In: NATO International Advanced Study Institute on Biological Effects of Visible Light. Photo- chemistry and Photobiology of “Photodynamic.Action." Univ. of Sassari, Sardinia, Italy, Sept. 3-17, 1969, pp. 108-109. Zelle, M. R. 1971. Genetic implications of fonizing radiation exposure. Abstract of invited talk in Plenary Session of 16th Annual Meeting of the Health Physics Society. Health Physics, 21: Supplement (abstracts) page 7. So Zelle, M. R. -11- Argonne National Laboratory Annual Reports: Kvetkas, M., and M. R. Zelle. 1966. Genetic analysis of a large cell strain of Escherichia coli. ANL-7278, pp. 162-164. , Rehfeld, C. E., M. R. Zelle, D. E. Doyle, J. A. Pagels, and M. H. Dipert. 1967. Analysis of familial influence on mean erythrocyte volume. ANL-7409, pp. 117-119. , Kvetkas, M. J., and M. R. Zelle. 1967. Genetic analysis of a large cell strain of Escherichia coli. ANL=7409, pp. 142-143. — Kvetkas, M. J., R. E. Krisch, and M. R. Zelle. 1968. Genetical and physiological studies of a large-cell, radiation-resistant strain of Escherichia coli. Part I. Genetic analysis. ANL-7535, pp.1-3. Kvetkas, M. J., R. E. Krisch, and M. R. Zelle. 1968. Genetical and physiological studies of a large-cell, radiation-resistant strain of Escherichia coli. Part II. Physiological studies. ANL-7535, pp. 3-5. Fritz, T. E., R. C. Zeman, and M. R. Zelle. 1969. Studies of the physiology of the Beagle thyroid gland. II. Pathology and familial incidence of thyroiditis in a closed Beagle colony. ANL-7635, pp. 107-108. ABSTRACTS OF PAPERS PRESENTED AT THE 1939 MEETINGS -OF THE GENETICS SOCIETY OF AMERICA Convscaus, Oxto, DECEMBER 28-30, 1939 E. W. “LINDSTROM, Secretary Deparimens of Genetics _ Towa Siaie College, Ames, Iowa 140 " Zexte, M. R., and Gowen, Joun W., Iowa State College, Ames, Iowa: On the origin of epidemic virulence. When an epidemic starts, a concatenation . of events occurs which enables a particular pathogen to sweep thruugit a spe- cies. To particularize these events constitutes a most significant problem of discase resistance. If host and pathogen are closely integrated, one must change in susceptibility or the other in invasive power if epidemic disease “proportions are reached. Following ScioTT’s experiments, our own have shown. ' that susceptibility or resistance in different host populations may be markedly increased by 25 generations of controlled breeding. But these changes are ton slow to explain the shift from a stable population to the unbalance of epi- demics. The immediate origin seems rather in mutation of the pathuzen’s virulence. The selective force isolating and purifying the population to the invasive type would lie in the hosts. Four experiments with mouse ty- phoid, S. aetrycke, support this analytical view. In experiments rz, 2 and 4 the host type did not materially influence virulence. The third experiment was markedly different. Bacteria, passed through: resistant mice, suddenly be- ‘came the most virulent experienced; those through the susceptible mice showed no such change. Reversal of hosts caused the very slightly virulent strain, previously inhabiting the susceptible mice, to gain virulence explosively in the third passage through resistant mice. The virulent strain from resistant mice, passed through the susceptibles, remained stable. The results thus far indicate infrequent pathogenic mutation with host selection as a cause of epi- demic virulence, agreeing with work of WELLHAUSEN (1937) and Lrncoty (3939) on the corn bacterial wilt relationship. ABSTRACTS OF PAPERS PRESENTED AT THE 1940 MEETINGS OF THE GENETICS SOCIETY OF AMERICA _ PHILADELPHIA, Pa., DECEMBER 30-JAN. 1, 1948 E. W. LINDSTROM, Secretary Department of Genetics Toews State College, Ames, Iowa 174 ZELLE, Max R., and Gowen, Joun W., Iowa State College, Ames, Iowa: Interaction of the genetic constitutions of host and pathogen in mouse typhoid—— Among the factors responsible for the inception of epidemic disease, the sud- ' den appearance of a highly virulent pathogen in a suceptible community is of first importance. The findings of these investigations indicate that high viru- Jence results from the. rare occurrence of a more pathogenic mutant within the bacterial population. Selection of the virulent form in the host results in the _ virulent form replacing the avirulent. The environments within genetically re- . sistant and susceptible mice are equally favorable to the virulent bacteria. . Four experiments, in each of which the same initial ‘culture of Salmonella typhimurium was passed through both resistant and susceptible mice, uphold ‘his viewpoint. In the first two experiments the original culture was a mixture of bacterial types differing in virulence. Increases in virulence occurred in both resistant and susceptible mice. The initial culture for the third experiment was a single cell culture of low virulence and slightly rough colony type. Ten passages through susceptible hosts resulted in no change in virulence. After one passage through resistant mice, virulence changed suddenly to sig- “nificantly higher virulence. Examination of the resulting bacterial population showed it ofipredominately smoother colony type than its parent. Particular . Strains of virulent and avirulent bacteria may be classified by their phenotypic appearance. The differential replacement of the avirulent by the virulent bac- teria has been checked by introducing known mixtures of the two types into susceptible.and resistant hosts. t ‘ i ABSTRACTS OF PAPERS PRESENTED AT THE 1940 MEETINGS OF THE GENETICS SOCIETY OF AMERICA PHILADELPHIA, Pa., DECEMBER 30-JAN. I, 1941 E. W. LINDSTROM, Seerelary , Depariment of Genetics Towa State College, Ames, Iowa 174 ZELLE, Max R., Iowa State College, Ames, Iowa: Ov the mechanism of variegation in. bacterial colonies——An unstable smooth colony culture of Salmonella typhimurium exhibiting a high mutation rate to a stable rough type has been studied by a technique designed to demonstrate that, at least in this instance, the SR transformation is a discontinuous change. The parent strain is a single-celled, smooth organism whose progeny are invariably mixtures of smooth and rough types. Rough colonies from this mixture give only rough. Employing a modified micromanipulator technique, single cells from a'smooth colony are seeded upon a thin film of agar mounted on a cover slip 0% « moist chamber. After division the two daughter cells are separated by a microneedle and their location marked. Upon further division the separation is repeated until ultimately the cells are allowed to develop microcolonies which are trans- ferred to broth, incubated, and plated. If the original single cell was smooth, the ultimate single cell‘ progeny cultures derived from it exhibit both colony types. If the ultimate single cell progeny isolated is a rough mutation then only rough colonies develop. Two mutations have occurred in 302 single-cell isolations of this type giving a mutation rate of 0.0066 per cell generation. In one of these isolates its sib-cell was inviable; in the other its sib-cell was smooth. Considering only the manner in which the changes occur, the simplest hypothe- sis is that the bacteria are haploid with a high mutation rate for the particular locus. However, the significantly higher frequency of inviable cells among the smooth bacteria indicate a more complex mechanism.