Differing roles for [Ca2+](i) transients in FcγR-mediated phagocytosis have been suggested based on the observations that antibody-opsonized erythrocyte phagocytosis by human neutrophils shows a [Ca2+](i) dependence, while that by murine macrophages appears [Ca2+](i)-independent. To explore whether this difference might reflect different receptor isoforms or different cell types, we studied the [Ca2+](i) dependence of receptor- initiated phagocytosis by human FcγRIIa and a panel of FcγRIIa cytoplasmic domain mutants expressed in murine P388D1 cells and by human FcγR endogenously expressed on human neutrophils and monocytes. Wild-type and point mutants of huFcγRIIa stably transfected into murine P388D1 cells have different capacities to initiate a [Ca2+](i) transient, which are closely correlated with quantitative phagocytosis (r = 0.94, p < 0.0001). Phagocytosis both by huFcγRIIa in P388D1 cells and by huFcγRIIa endogenously expressed on neutrophils and blood monocytes shows [Ca2+](i) dependence. Phagocytosis of antibody-opsonized erythrocytes by neutrophils demonstrated greater susceptibility to [Ca2+](i) quenching compared with FcγRIIa-specific internalization with E-IV.3, suggesting that the phagocytosis activating property of FcγRIIIb in neutrophils also engages a [Ca2+](i)-dependent element. In contrast, phagocytosis by human FcγRIa, endogenously expressed on blood monocytes, is [Ca2+](i)-independent. Despite the importance of a consensus tyrosine activation motif for both receptors, FcγRIa and FcγRIIa engage at least some distinct signaling elements to initiate phagocytosis. The recognition that both of the phagocytic receptors on murine macrophages and human FcγRIa associate with the FcεRI γ-chain, which contains a tyrosine activation motif distinct from that in the FcγRIIa cytoplasmic domain, suggests that [Ca2+](i)- independent phagocytosis is a property associated with the utilization of γ- chains by FcγR.