The role of murine IL-1β in vitro and in vivo has not been defined. We describe here the production of neutralizing and immunoprecipitating mAb and polyclonal antibodies specific for murine IL-1β and their application to a characterization of the murine IL-1β protein. Immunization of either hamsters or rabbits with the recombinant mature form of murine IL-1β emulsified in CFA elicited antisera and hamster mAb that only recognized denatured IL-1β. In contrast, immunization with rIL-1β adsorbed to alum resulted in the generation of neutralizing and immunoprecipitating rabbit and hamster antisera and hamster mAb. All of the mAb recognize both the pro-form of IL-1β and the mature bioactive form produced by cultures of murine peritoneal macrophages. Using these antibodies, we demonstrate that approximately half of the IL-1 activity present in supernatants of LPS-treated cultured mouse macrophages is composed of IL-1β. Additionally, IL-1β as well as IL- α can be detected in the plasma of LPS-treated mice. These studies, therefore, demonstrate the production of IL-1β both in vitro and in vivo.