Mice were immunized with picryl chloride and the regional nodes taken at various times afterwards. These cells spontaneously synthesized DNA in vitro as measured by thymidine incorporation over an 18-hour period and the peak incorporation occurred when the cells were taken on day 3. When the mice were injected with cells taken 5 days after immunization with picryl chloride and then immunized, there was a depression of the spontaneous DNA synthesis in vitro. This was absent on day 2, most marked on day 3 and still present on day 4. Cells from donors immunized with 4-ethoxymethylene-2-phenyloxazolone had a smaller but definite effect. Attempts to reproduce the phenomenon by in vitro mixtures of cells taken at various times after immunization in vivo were unsuccessful.