The capacity of rat peritoneal macrophages to produce interleukin -1β (IL-1β) following phagocytosis of latex particles in vivo and in vitro was examined. In both cases, a marked increase in IL-1β secretion was observed, although the level of the cytokine secreted in vivo was higher than that observed after incubation of the cells with latex beads in vitro. It is presumed that this difference is due to stimulation of the peritoneal macrophages by endogenous produced factors/cytokines prior and during phagocytosis in vivo. Macrophages stimulated with LPS showed a level of IL-1β almost identical to that obtained after incubation with latex. Following phagocytosis in vivo and further stimulation with LPS in vitro, the cells showed an additional increase in IL-1β production, whereas this additive effect could not be observed when incubation with both latex and LPS was carried out in vitro. The results suggest different patterns for IL-1β production by rat peritoneal macrophages, depending on the way they are stimulated for phagocytosis.