The responsiveness of adult mouse thymocytes to interleukin 1 (IL-1) was investigated. When stimulated with suboptimal concentrations of ionomycin and phorbol myristate acetate (PMA), the immature subpopulation of Lyt2-,L3T4- (2-4-) thymocytes responded to exogenous, purified IL-1 in a dose-dependent manner. In contrast, mature lymph node (LN) T cells were unresponsive to exogenous IL-1 under similar conditions. The proliferation of 2-4- thymocytes, which comprise 2-3% of cells in the adult mouse thymus, was independent of exogenous IL-2 and accounted for essentially all the 3H-thymidine incorporation by unfractionated thymocytes in response to IL-1. Optimally stimulated 2-4- cells did not show responsiveness to exogenous IL-1, and 48 h supernatants from these cultures were found to contain IL-1 like activity. Taken together, the data point to a role for IL-1 in the proliferation of immature 2-4- cells in the thymus.