Human neutrophils express two structurally distinct receptors for IgG, the transmembrane FcγRII (CD32) and the glycosyl-phosphatidylinositol-linked FcγRIII (CD16). To explore the respective functional roles of FcγRII and FcγRIII, we have used anti-receptor mAb fragments coupled with erythrocytes to quantify both individual and cooperative receptor functions. With individual receptor engagement, FcγRII (E-IV.3) was much more efficient than FcγRIIIB (E-3G8) in initiating phagocytosis (p < 0.001). However, when identical total numbers of receptors were engaged, co-ligation of FcγRII and FcγRIIIB resulted in a phagocytic response, which was: 1) greater than that for either receptor alone (>twofold FcγRII alone (p < 0.001) and >20-fold FcγRIIIB alone (p < 0.001)); 2) greater than the sum of FcγRII and FcγRIIIB (p < 0.001); and 3) comparable to the phagocytic potential of FcγRII in FMLP pre-activated neutrophils. This synergistic capacity of FcγRIIIB also enabled CR phagocytosis. Furthermore, the capacity for FcγRIIIB to interact synergistically with FcγRII was preserved in FMLP- preactivated neutrophils. The activation of FcγRII by FcγRIIIB was associated with tyrosine phosphorylation of the FcγRII cytoplasmic domain, which is essential for FcγRII function and which, by analogy to the Ig α- chain of the B cell Ag receptor complex, the ζ-chain of CD3, and the γ- chain of some FcεRs and FcγRs, may enhance the binding and activation of Src or Syk family tyrosine kinases. Thus, FcγRIIIB can affect multiple receptor families and play a role in achieving maximal FcγR capacity, even in stimulated neutrophils in inflammatory sites.