Studies with populations of macrophages have produced conflicting results concerning the possibility that the concentration of intracellular ionized calcium [( Ca2+]i) may act as an important mediator for phagocytosis. Since asynchronous changes in [Ca2+]i in individual cells undergoing phagocytosis may be averaged to undetectability in population studies, we studied single adhering murine macrophages using fura-2 and our previously described digital imaging system. The proportion of macrophages phagocytosing IgG-coated latex beads was greater than for uncoated beads (percent phagocytosing cells: 71 +/- 7 vs. 27 +/- 7, P less than 0.01). Phagocytosis of IgG-coated and uncoated beads was always associated with a calcium transient that preceded the initiation of phagocytosis. No calcium transients were detected in cells that bound but did not phagocytose beads. Four major differences between Fc receptor-mediated and nonspecific phagocytosis were detected: (a) the duration of calcium transients was longer for nonspecific phagocytosis compared with Fc receptor-mediated phagocytosis (69.9 +/- 10.2 vs. 48.7 +/- 4.7 s, P less than 0.05) and the magnitude of calcium transients was less for nonspecific phagocytosis (178 +/- 43 vs. 349 +/- 53 nM, P less than 0.05); (b) removal of extracellular calcium abolished the calcium transients associated with nonspecific phagocytosis but had no effect on those associated with receptor-mediated phagocytosis; (c) in the absence of extracellular calcium, buffering intracellular calcium with a chelator reduced Fc receptor-mediated phagocytosis but had no additive inhibitory effect on nonspecific phagocytosis; and (d) inhibition of protein kinase C (PKC) with staurosporine inhibited nonspecific phagocytosis but had no effect on receptor-mediated phagocytosis. Our observations suggest that despite both types of phagocytosis being associated with intracellular calcium transients, the role played by intracellular calcium in the signaling pathways may differ for Fc receptor-mediated and nonspecific phagocytosis by elicited murine macrophages.