Earlier studies have shown that the intracellular killing of Staphylococcus aureus by human monocytes requires continuous stimulation by serum factors, e.g., immunoglobulin G (IgG). In the present study, we demonstrate that IgG, at concentrations that stimulate the intracellular killing of S. aureus, induces a transient increase in the intracellular free calcium concentration ([Ca2+]i) in monocytes. The Ca2+ ionophores A23187 and ionomycin stimulate the killing process as efficiently as IgG does and initiate O2- production in resting monocytes but not in monocytes containing bacteria. The Ca2+ ionophore-stimulated killing process was markedly inhibited by the NADPH oxidase inhibitor diphenyleneiodonium bisulfate, which indicates that these ionophores stimulate oxygen-dependent bactericidal mechanisms. Reduction of the [Ca2+]i to values below 1 nM, obtained by loading monocytes with MAPT/AM (1,2-bis-5-methyl-aminophenoxylethane-N,N,N',N'-tetraacetoxymet hyl acetate) in the absence of extracellular Ca2+, rendered the cells unresponsive to IgG or Ca2+ ionophore stimulation of the intracellular killing of S. aureus, but the response could be restored by reincubating these cells in the presence of extracellular Ca2+. It is concluded that cytosolic free Ca2+ is essential for the IgG-stimulated intracellular killing of S. aureus by human monocytes.