Department of Genetica
University of Wisconsin
Madison 6, Wieconsin, U.S.

May 4, 1951.

Dr. de 3S. Ke Boyd,
Wellcome Laboratories of Tropical Medicine,
London, England.

Dear Br. Boyd:

I read your paper on "The Symbiotic Bacteriophages of Salmonella
typhi-murium" (J. Path. Bact. 62:501-517) with great interest. Inci-
dentally to genetic studies along rather different lines, one of my
students (Mr. Zinder) has had oceasion to study the type cultures of
bttinkt?t Lilleengen's collection with respect to their lysogenicity.

It was very soon apparont that tho resistance patterns were dependent

on the set of lysogenic phages present rather more than upon the intrinsic

or genotypic resistance patterns. In additon, it was possible to alter

the apparent phage type of many of the cultures by superinfecting

them with phages from other cultures. This might be as much of a complication
in typing on the basis of the symbiotic phages as on the resistance patterns
but the former is certainly technically more satisfactory.

Would it be possible for you to send us two reprints of this paper?
I would also appreciate the favor if we could have the indicator strains
(1404 and 1411) used in your studies. Like yourself, we have had some
difficulty in collecting likely uninfected cultures, and are not satis-
fied with the indicators presently available.

May I enticipate your assistance on thie matter with my thehke,
Sincerely,

Joshua Lederberg,
Associate Professor of Genetics