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