The cytokine interleukin 1 (IL 1) plays an important role in the induction of IL 2 secretion and high-affinity IL 2 receptor (IL 2R) expression by T cells. The events that follow binding of IL 1 to IL 1R, however, are still unknown. In this study we describe two variants of the murine thymoma EL4 (5D3 and D6/76) that express comparable numbers of cell surface IL 1 receptors and bind IL 1 with the same affinity, but show distinct IL 1-dependent IL 2 secretion and IL 2R expression. In the presence of the tumor promoter phorbol 12-myristate 13-acetate IL 1 augments IL 2 secretion and IL 2R expression of EL4 5D3 but not of EL4 D6/76 cells. Comparison of the internalization of IL 1 by both clones revealed that EL4 D6/76 was unable to transport cell surface-bound IL 1 to the cytoplasm. These findings suggest that internalization of receptor-bound IL 1 is required for the action of this cytokine.