A simple and reliable assay for interleukin-1 (IL-1) is described which has the advantage over other assays that it is independent of interleukin-2 (IL-2) production by the test cells. The assay makes possible the detection of IL-1 in the supernatants of T cell populations. The ability of IL-1 to induce IL-2 receptor expression in the absence of T cell mitogen is the basis of this assay. Thus, proliferation of mouse thymocytes incubated in the presence of saturating concentrations of IL-2 (10 U/ml) was directly dependent on the concentration of IL-1. The sensitivity of the assay is comparable to the sensitivity of the classical thymocyte co-stimulator assay. Natural and recombinant human and murine IL-1 were measured in this test system with comparable sensitivity.