Numerous mAbs have been generated against Fc gamma RIII (CD16), the low-affinity receptor for the Fc part of IgG. Most of the mAbs recognize both the receptor isoforms, transmembranous Fc gamma RIIIA, and glycosylphosphatidylinositol-linked Fc gamma RIIIB. Binding epitopes of some of the mAbs that differentiate between the two neutrophil Ag (NA) alleles of Fc gamma RIIIB (CLB-Gran11 against NA1; GRM1, BL-LGL/1 against NA2 allele) have been mapped on the first, membrane-distal domain of CD16. We demonstrate that mAbs 3G8, B88-9, CLB-Gran1, MEM-154, and LNK16 almost completely block the receptor's interaction with IgG. Using chimeric Fc gamma RIIIB/Fc epsilon RI receptors and molecular modeling, we localized the epitopes of 3G8 and B88-9 on the putative FG loop of the membrane-proximal Ig-like domain, which we have previously identified as the major binding site for IgG. The epitopes of CLB-Gran1 and MEM-154 are shown to reside in proximity to the FG loop (probably BC or C'E loop). The blocking mAb LNK16 was detected to be directed against the putative C' beta-sheet of the membrane-proximal domain, suggesting that additional residues may be involved in IgG binding of Fc gamma RIII.