Interleukin 1 (IL-1) is a polypeptide hormone that mediates a broad range of biological activities and interacts with surface receptors on numerous cell types. Equilibrium binding studies have identified a class of IL-1 receptors on T cells, fibroblasts, and epithelial cells that have 2- to 5-fold higher affinity than the receptors on bone marrow cells, pre-B cells, and macrophage cell lines. Affinity cross-linking with human 125I-labeled IL-1 alpha (125I-IL-1 alpha) labels an approximately 100-kDa protein on T cells and fibroblasts and an approximately 80-kDa protein on pre-B cells and macrophage cell lines. Monoclonal and polyclonal antibodies specific for the IL-1 receptor on T cells and fibroblasts block human 125I-IL-1 alpha binding to T cells, fibroblasts, and epithelial cells but cannot block IL-1 binding to bone marrow cells, pre-B cells, and macrophages. These antibodies immunoprecipitate the IL-1 receptor-human 125I-IL-1 alpha complex from T cells and fibroblasts but not from pre-B cells and macrophage cell lines. An S1 nuclease protection assay demonstrated that T cells and fibroblasts contain identical IL-1 receptor mRNA but that pre-B cells and macrophages do not contain this receptor mRNA. Taken together, the data demonstrate that mouse T cells, fibroblasts, and epithelial cells express an identical IL-1 receptor, whereas the IL-1 receptor on pre-B cells, macrophages, and bone marrow cells represents a different gene product.