Human polymorphonuclear leukocytes (PMN) express two classes of FcγR; FxγRII the 42-kDa receptor with a traditional membrane spanning domain and cytoplasmic tail and FcγRIII(PMN) the 50- to 80-kDa receptor with a glycosyl-phosphatidylinositol membrane anchor expressed on PMN. To explore the capacity of FcγRIII(PMN) to generate intracellular signals, we have analyzed the ability of Fab and F(ab')2 anti-FcγR mAb to induce actin filament assembly, a prerequisite for motile behaviors. Multivalent ligation of FcγRIII(PMN), independent of FcγRII, results in an increase in F-actin content that is [Ca2+](i) dependent. Multivalent ligation of FcγRII also initiates actin polymerization but uses a [Ca2+](i)-independent initial pathway. In addition to providing a mechanism for FcγRIII(PMN) triggered effector functions, the increase in F-actin and [Ca2+](i) generated by FcγRIII(PMN) ligation also serves as a 'priming' signal to modify PMN responses to other stimuli. Experiments using erythrocytes specifically coated with antiFcγRII Fab demonstrate that cross-linking of FcγRIII(PMN) with anti-FcγRIII F(ab')2 enhances phagocytosis mediated by FcγRII. Thus, FcγRIII(PMN), a glycosyl-phosphatidylinositol anchored protein, may contribute directly to an intracellular program of actin assembly that may trigger and prime neutrophil effector functions.