April 13, 1949.

_ Dear Aaron,

Strain 58 (biotinless, from K-12) has been sent under separate cover.
Along with it I enclosed two phages, p20 and pda isolated from the sewage
you sent me. These interested me because lambda interferes comletely
with them. p20a 1s 4 plaque—type mutant which has been noticed once from
p20. I would appreciate it if you could look at thea on B and tell me
whether they differ in the same way that r mutants appear to. These phages
show complete cross-resistance with T6, which 1s not, however, interfered
with by lambda. However, a mitant (frequency ca. 1076) can be obtained
from p20 which I cannot distingusih from T6, as it mckamix pverrides the
interference barrier. On the other hand, genetic resistance to T6, i.e.
the sutation Ves does not influence the maintenance of lambda, and some
lamtida~resistant stocks that I have, are sensitive to p20. The plaque-type
mutation and the mitation for overriding lambda seem to be independent of
each other and may be good material for recombination studies, and I my
try my hand at it.

If you can manage to be here May 7 (Sat. PM) we have a set of tickets
for 4 matinee performance of Man and Superman with Maurice Evans, and
Sachs and we will be able to go together. Please let us know whether to
hold on to the ducatas

You will probably have heard this more directly, but Kelner is on the
loose, with prospective backing of a fellowship. We would like to, but don't
have the space to accomodate him here. Wouldn't the Institute be a good bet?

Hope to see you soon, Sincerely,