Interleukin 6 (IL-6) and interleukin 1 (IL-1) were compared as to their ability to induce the proliferation of distinct thymocyte subpopulations. IL-6 functions as a costimulator in IL-1- or IL-2-induced proliferation of adult double-negative (DN) thymocytes, whereas IL-6 alone failed to induce a significant level of proliferation. However, IL-6 alone induced significant proliferation of mature cortisone-resistant thymocytes, whereas IL-1 did not. Instead, IL-1 functioned as a co-stimulator in IL-6-induced proliferation of mature thymocytes. Finally, both IL-6 and IL-1 were capable of potentiating IL-2-induced proliferation of fetal DN thymocytes. These data suggest that the monokine requirements in thymocyte activation may vary during thymocyte maturation and that IL-6, when compared to IL-1, has a distinct effect on the proliferation of thymocytes.