One of the biological activities of interleukin 1 (IL1), the lymphocyte-activating function, is usually quantitated by measuring the effect on thymocyte proliferation in the presence of a T cell mitogen. Tumor necrosis factor (TNF) is also produced by activated macrophages, and it has recently been shown that it can enhance the proliferation of mature, human T cells. To analyze the role of these macrophage products in the proliferative response of murine thymocytes, we have tested the effects of purified (produced by the recombinant DNA techniques) IL1 and TNF in thymocyte cultures, where the cell concentration, mitogen as well as the accessory cell amount was varied. The data obtained show that both IL1 and TNF can facilitate the thymocyte proliferation, but the effect of TNF required a stronger T cell activation signal (a higher phytohemagglutinin, PHA, dose or concanavalin A instead of PHA) to become optimal. Moreover, the effect of IL1 was totally abolished by depleting the Ia+ accessory cells from the thymocytes, while that of TNF remained intact. IL1 and TNF acted synergistically in this activation and this effect was so strong that thymocyte proliferation started even in the absence of the mitogen. Thus, the data suggest that these cytokines act on different steps of the T cell activation process.