May-6, 1949.

Dear Frantz,
. Thunks very mich for the ganerous samples of dactdbionate.

You cartainly hit the nail right on the head when you pointed out
the difficulty of excluding lactose as the eyocator ef the adaptive
response. Lactosa is very active: M/5000 still gives nearly optim
_Fesponse, and it wouldn't take a great deal of contaminaticn of lacto-
bicnute $o approach this concentration in the M/100 soluticns I had |
been using. Tho mre purified praps. are much less active, but there is
still some residual antivi ty which may or may not be due to lactose,
perhaps potaatated by Jagtohionate. But it will take a good deal more
study to settle the question entirely. I think that I should be able to
eliminate lactose entirely, for all practical purposes, by differential
fermentation! I lat you know if anything new pops up.

“Although this prospect has largely fizzled, there is 4 confirmed —
differential in the activity of lactose and butyl galactoside. In the
Lac,- mitants » lactcse evowes a tenfold lower level of lactase than does

- the galactpside. Thus, cells grown on galachoside ferment lactose very
mach more repidiy than those grown on lactose itself.

| Sincerely,

Joshua Lederberg