Recombinant human interleukin 1 receptor antagonist (IL-1ra) and 35F5, a neutralizing monoclonal antibody (mAb) to the type I mouse IL-1 receptor, were examined for their ability to bind to IL-1 receptors (IL-1Rs) on various types of mouse cells and to block immune and inflammatory responses to IL-1 in vitro and in mice. IL-1ra competed for binding of 125I-IL-1 alpha to type I IL-1R present on EL-4 thymoma cells, 3T3 fibroblasts, hepatocytes, and Chinese hamster ovary cells expressing recombinant mouse type I IL-1R. The IC50 values for IL-1ra binding (ranging from 2 to 4 ng/ml) were similar to those of IL-1 alpha. In contrast, IL-1ra bound with very low affinity (IC50 values ranging from 10 to 200 micrograms/ml) to cells expressing type II IL-1R, i.e., 70Z/3 pre-B cell line and polymorphonuclear leukocytes (PMN) derived from bone marrow and acute inflammatory exudates. The mAb 35F5 bound specifically to type I IL-1R; no inhibition of 125I-IL-1 alpha binding to cells having type II IL-1R was observed with very high concentrations of antibody. While neither IL-1ra nor 35F5 had intrinsic activity in bioassays using T helper D10.G4.1 cells and mouse thymocytes, both agents blocked the ability of IL-1 to stimulate proliferation of these cells. The effects of IL-1ra and 35F5 on acute inflammatory responses in mice were also evaluated. IL-1ra and 35F5 blocked the local accumulation of PMN after intraperitoneal injection of rIL-1 alpha. The response to IL-1 was inhibited when IL-1ra or 35F5 was administered simultaneously with or before administration of IL-1. IL-1ra and 35F5 also blocked PMN accumulation after intraperitoneal injection of lipopolysaccharide or proteose peptone, suggesting IL-1 is important in mediating responses to these agents. In addition, IL-1ra and 35F5 significantly blocked the ability of IL-1 to stimulate egress of PMN from bone marrow, to induce a transient neutrophilia, and to elevate serum levels of hepatic acute phase proteins, IL-6, and corticosterone. Thus, IL-1ra and 35F5 competitively inhibit the binding of IL-1 to the IL-1R on certain cell types. These two IL-1 receptor antagonists act to inhibit biological responses induced by IL-1 and other inflammatory agents.