In vitro phagocytosis of IgG-opsonized sheep erythrocytes (EA) was used to measure the in vivo activation of mouse peritoneal macrophages. Uptake of EA as enhanced by the extraperitoneal administration of Newcastle disease virus, vesicular stomatitis virus, tilorone or polyinosinic-polycytidylic acid. Ingestion of EA was similarly stimulated by lipopolysaccharide or killed Corynebacterium parvum. Dose-response curves relating concentrations of IgG to phagocytosis were parallel for both treated and control animals. This indicates that the heterogeneity of the macrophage populations did not change and that the overall populations were activated with respect to phagocytic ability. Numbers of macrophages were not increased (except in C. parvum-treated mice), suggesting that resident, rather than newly recruited macrophages, were activated by the different agents.