To investigate the ability of FcγRIII(PMN), the GPI-anchored isoform of FcγRIII (CD16) in polymorphonuclear leukocytes (PMN), to mediate transmembrane signaling events, we measured changes in membrane potential with DiOC(5) and in intracellular calcium with indo-1. FcγR were ligated by anti-FcγRIII mAb 3G8 (IgG and Fab), anti-FcγRII mAb IV.3 (IgG and Fab), and human IgG aggregates. Cell bound mAbs were also crosslinked by goat F(ab')(2) anti-mouse IgG. 3G8 IgG elicited a rapid change in [Ca(2+)](i), which was unaffected by EGTA, Vibrio cholerae toxin (CT), or Bordetella pertussis toxin (PT), and was abolished by BAPTA . Univalent receptor binding with 3G8 Fab gave no response but crosslinking with F(aV)2 GAM gave a rapid [Ca2,](i) response. Neither IV.3 Fab, IV.3 IgG, nor crosslinking of IV.3 Fab elicited a calcium signal. PI-PLC-treated PMN with the density of FcγRIII(PMN) reduced to that of FcγRII showed an unattenuated change in [Ca(2+)](i), with a 3G8 stimulus. The effects of IgG aggregates paralleled those of 3G8 mAb. These data indicate that multivalent ligation of FcγRIII(PMN) initiates an increase in [Ca(2+)];, derived from intracellular stores, that is distinct from both the FMLP- and FcγRII-induced responses. Ligand-dependent interaction with FcγRII is not required. Since FcγRIII(PMN) can internalize the FcγRIII-specific probe Con A-opsonized E and lyse anti-FcγRIII heteroantibody-opsonized chick E, this GPI-anchored molecule mediates both signal transduction and integrated cell responses.