February 25, 1955

Dr, Clifford Grobstein
National Cancer Trstitute
Bebhesda 14, Maryland

Dear Cliff:

I thought you might like to know how our experiments came out on
the trunamissibility of "F" through the thin millipore filters.

Esther and I did severaj runs, including some dry runs with the
standard .150 umn. filters. The results were quite definitely negative:
whenever the F- culture had become F+ there had also teen a breakthrough
of the F+ cells themselves. Technically, it worked out quite well: I had
a fair proportion of break throughs when I smeared fairly large areas,
but these vers readily diagnosed by the differential markers afterwards.
We also ahecked that cultures growing on the same side of a filter, top
or bottom, exchanged FP quite readily.

We alec have some experiments on trying to cross Hfr x F~ through a thin
filter; again, no success unless intact cells (i.e. markar combination-) had
demonstribly gotten through.

We did not begin to use up all the filter material you generously pro-
vided. I would like to have the residue for odd experiments that may coms
up, but if you are pressed, will be happy to return it. If you have occasion
somtime to reorder a batch of the thin stuff, would you let me know an’
let us join in your order?

Sterilization turned out to be no problem at all. UV was tried, and probably
would work, put turned out to be superfluous, as the un-UV'd samples wera
all yuite sterile (on EMB lactose medium).

Ses you next month,

Sincerely,

fof -
| PULA a
//Joshua Lederberg

P.S. We have some other evidence in
support of the idea that F transfer is
normally related to direct cell to cell
contact.