A method has been described for obtaining radioautographs of plaque-forming cells. The method permits radioautographic analyses of small numbers of plaque-forming cells amidst large populations of non-plaque-forming cells. Spleen cells that were pulse-labeled with tritiated thymidine could be categorized readily as labeled or not labeled.

Using this method it was found that (a) at least 55% of plaque-forming cells which appear 3 days after a maximal stimulus of 4 x 108 sheep red cells are still capable of DNA synthesis, and must have arisen by cell proliferation; (b) the rate of proliferation of plaque-forming cells is proportional to the log of the dose of antigen; (c) the S period of plaque-forming cells is at least 2 hr, appears to be constant, and is not influenced by antigen dose. The results suggest that antigen stimulates proliferation of plaque-forming cells by hastening their transit through the G1 phase of the generative cycle.

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