The cytokine interleukin-1 plays an important role in the induction of IL-2 secretion and IL-2 receptor expression. The events that follow binding of IL-1 to its receptor are not known. We found that in a purified T cell population (comprising the Lyt2- and L3T4- T cells) IL-1 in the absence of antigen or mitogen induced strong proliferation and de novo expression of IL-2R light chain mRNA. This was accompanied by high affinity IL-2 binding. Production of IL-2 or IL-2 mRNA was not detected under these conditions. As a model system for IL-1 action two EL4 subclones were isolated. EL4 5D3 responded to IL-1 by augmentation of PMA induced IL-2 secretion and IL-2R expression. EL4D6/76 bound IL-1 with the same affinity but did not respond. We found that this line was unable to internalize surface bound IL-1. The finding suggests that in T cells internalization of IL-1 is required for its activity.