We report the production of hamster anti-murine IL-1α mAb and analysis of their specificity and suitability for use in murine IL-1 immunologic and biologic assays. The mAb bound to murine (Mu) rIL-1α3 but not rMuIL-1β as assessed by both direct ELISA and immunoprecipitation. They also inhibited the biologic activity of MuIL-1α but not the activity of rMuIL-1β as measured in a T cell co-stimulator assay. These IL-1α specific mAb only partially inhibited the co-stimulator activity found in macrophage culture supernatants but completely inhibited the co-stimulator activity of fixed peritoneal exudate cells. The data indicate that the species of IL-1 associated with murine macrophage membranes shares at least two epitopes with IL-1α and probably represents a product of the IL-1α gene. These reagents will be valuable for quantitative assessment of specific IL-1 proteins on cell surfaces, in culture supernatants, and in cell lysates. They will also be useful both in vitro and in vivo for determining the relative roles of the different IL-1 species in the development of biologic responses.