Rises in intracellular-free calcium ([Ca(2+)](i)) have been variously associated with Fcgamma receptor (FcR)-mediated phagocytosis in macrophages. We show here that activation of murine bone marrow-derived macrophages increases calcium spiking after FcR ligation. Ratiometric fluorescence microscopy was used to measure [Ca(2+)](i) during phagocytosis of immunoglobulin G (IgG)-opsonized erythrocytes. Whereas 13% of nonactivated macrophages increased [Ca(2+)](i) in the form of one or more spikes, 56% of those activated with lipopolysaccharides (LPS; 18 h at 100 ng/ml) and interferon-gamma (IFN-gamma; 100 U/ml) and 73% of macrophages activated with LPS, IFN-gamma, interleukin (IL)-6 (5 ng/ml), and anti-IL-10 IgG (5 micro g/ml) spiked calcium during phagocytosis. Calcium spikes were inhibited by thapsigargin (Tg), indicating that they originated from endoplasmic reticulum. The fact that activated macrophages showed a more dramatic response suggested that calcium spikes during phagocytosis mediate or regulate biochemical mechanisms for microbicidal activities. However, lowering [Ca(2+)](i) with ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid or inhibiting calcium spikes with Tg did not inhibit phagosome-lysosome fusion or the generation of reactive oxygen or nitrogen species. Thus, the increased calcium spiking in activated macrophages was not directly associated with the mechanism of phagocytosis or the increased antimicrobial activities of activated macrophages.