We describe a newly recognized 40 kD FcR for IgG on human neutrophilic granulocytes. An mAb (IV3) developed against the IgG FcR of K562 cells, and specific as well for a 40 kD FcR on human monocytes and platelets, was found to purify by affinity adsorption a 40 kD protein from detergent lysates of surface-radioiodinated neutrophils. This protein, proteolytically degraded to 33 kD when purified in the absence of diisopropylfluorophosphate, is distinct from the 51-73 kD protein precipitated by the anti-neutrophil FcR mAb, 3G8, previously described by others. Complete inhibition of binding of rabbit IgG-coated erythrocytes to neutrophils was achieved only when both antibodies, IV3 and 3G8, were used. Fab fragments of IV3 were as effective inhibitors as the intact molecule. IV3 IgG or Fab fragments completely and selectively inhibited immune complex-mediated generation of superoxide by human neutrophils; superoxide generation by other stimulants was not abrogated by IV3. This antibody (IV3) bound also to human eosinophils and completely inhibited the binding of IgG-coated erythrocytes to eosinophils. IV3 appears to define the human homolog of the murine macrophage FcRII identified initially by mAb 2.4G2 and present in the mouse on both neutrophils and eosinophils.