This study investigates the role played by Fc gamma RI and Fc gamma RII in triggering the respiratory burst induced in human monocyte-like U937 cells by monoclonal chimaeric anti-NIP antibodies expressing the human IgG subclass and mouse IgG2b heavy chains. Respiratory burst activity was measured as superoxide generation. Four separate lines of evidence indicate a predominant role for Fc gamma RI in triggering superoxide generation induced by erythrocytes sensitized with up to the maximum of 100,000 IgG molecules per cell. Firstly, erythrocytes sensitized with mouse IgG2b anti-NIP antibodies which are not recognized by human Fc gamma RI, did not induce a response but when residue Glu-235 was replaced by Leu to give the lower hinge sequence of mouse IgG2a which is recognized by Fc gamma RI, the mutant bound to Fc gamma RI and induced a response equal to 80% of that given by chimaeric human IgG3. Chimaeric human IgG3 antibodies with amino acid substitutions in the lower hinge showed reduced activity and the greatest reductions (< 32% of wild type antibody activity) were associated with changes at Leu-235 which is critical for recognition by Fc gamma RI. Secondly, chimaeric human IgG4 antibodies which are not recognized by Fc gamma RII, were able to induce superoxide generation. The rank order of abilities of chimaeric human IgG subclass antibodies to induce responses was IgG3 > IgG1 > IgG4 > > IgG2. Thirdly, responses induced by chimaeric human IgG were inhibited by concns of monomeric human IgG3 in the nM range. Finally, chimaeric human IgG3 induced responses were inhibited by anti-Fc gamma RI, but not anti-Fc gamma RII monoclonal antibodies. Consistent with a major role for Fc gamma RI in triggering the responses of U937 cells, erythrocytes sensitized with chimaeric human IgG3 did not induce superoxide generation by neutrophils which express Fc gamma RII and Fc gamma RIII, or eosinophils which express Fc gamma RII, but neither of which expresses Fc gamma RI.