Angiotensin II (At II) has been shown to inhibit in vitro the IgG2a-mediated rosette formation of 51Cr-sheep red blood cells (SRBC) by provoked peritoneal macrophages (PM) at 10 (-5)-10 (-6) M concentrations. The decreased rosette formation was associated with an increased phagocytosis. It was found that the enhanced rosette formation at 10 (-7) M hormone concentration was followed by diminished phagocytosis via Fc gamma receptors (R). Processes mediated through Fc microR were affected only after incubation with 10 (-5) M of At II. The attachment and subsequent phagocytosis through C3bR was markedly enhanced by At II in a dose-dependent way. Thus, the relative phagocytosis (RP) through both FcRs was significantly enhanced by 10 (-5), ie by 10 (-5)-10 (-6) M of At II, but lowered at 10 (-7) M hormone concentration. In addition, there was no RP enhancing effect of At II after preincubation with 10 (-5) M of indomethacin (IM), indicating the significance of prostaglandins (PG) in the hormone effect. The medium containing 5 mM of EGTA diminished both the RP enhancing and inhibiting effects of At II. The RP mediated by C3b was not affected by At II, IM or EGTA. The intracellular killing capability, measured by chromium release from Candida albicans, was not altered or even slightly diminished after At II treatment of PMs.