Interleukin-1 (IL-1) is a factor that can induce proliferation of murine T lymphocytes^1,2 and can elicit a variety of other biological responses. These include bone resorption³, fibroblast proliferation⁴, acute phase protein release from hepatocytes⁵, cartilage breakdown⁶ and fever⁷. This spectrum of activities is consistent with a role for IL-1 as a mediator of inflammation. Recently, sequence data have shown that there are at least two members of the IL-1 family; these distantly related proteins have been termed IL-1α and IL-1β^8–11. We have found previously that both murine T cells and fibroblasts possess a relative molecular mass (M_r) ∼80,000 (80K) plasma membrane receptor for human IL-1β^12,13. We show here that the receptor for IL-1α on both murine and human cells is identical to that for IL-1β. This result raises the issue of what separation, if any, there might be between the biological activities of IL-1α and IL-1β.