Activated cells of the monocyte—macrophage lineage produce two forms of the inflammatory cytokine interleukin-1 (IL-1), IL-1α and IL-1β, of which IL-1β is the predominant secreted form and has a wide range of modulatory effects on the endocrine system. Immunoassays of human IL-1β have been described, but are not suitable for measurement of rat and mouse IL-1β because of limited cross-reactivity. Polyclonal sheep anti-rat or sheep anti-mouse IL-1β antisera were used to develop sensitive and specific immunoradiometric assays for rat and mouse IL-1β. Secretion of IL-1β from endotoxin-activated monocytes or macrophages was measured in vitro or in vivo in both species. In vitro, rat monocytes and mouse macrophages produced IL-1β in response to endotoxin, with a relatively small proportion of total IL-1β being secreted. In vivo, endotoxin stimulated an increase in plasma IL-1β in both animals. The development of these assays will facilitate studies of the role of endogenous IL-1β in animal endocrine models.