The DNA synthetic response of a wide variety of parental thymocyte doses was studied in the lymphoid tissues of lethally irradiated F1 mice. The response curves of the thymocytes were strikingly similar in shape to those of T cells responding to antigens, such as sheep red cells, which are more labile than the histocompatibility antigens of the F1 host. The response was characterized by a dose-dependent latent period of 1–3 days, followed by a sharp increase in activity and a significant subsequent shutoff. Larger thymocyte doses tended to shorten the latent period. A comparison of the responses of different cell doses to one another indicated that the response usually simulated a suppressed first-order reaction. However, occasional multiple order reactions were observed. We have interpreted these data to indicate that both positive and negative interactions occur between thymocytes, without the mediation of B cell products such as conventional antibodies.

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