In the present study 3-iodo-4-hydroxy-5-nitrophenacetyl (NIP)-specific antibodies were compared for induction of antibody dependent lysis of NIP-derivatised red blood cells effected by pre-stimulated U937 or HL-60 cells and by K cells. The chimaeric antibodies have heavy chains corresponding to human IgG subclasses 1–4, and include site-directed mutants of IgG3 as well as the aglycosylated form of IgG3; a mouse IgG2b antibody and a site-directed mutant IgG2b were also examined. rIFN stimulated U937 or HL-60 cells express increased levels of Fcγ R1 compared to unstimulated cells; PMA stimulated HL-60 and U937 cells express an increased level of Fcγ R11 compared to unstimulated cells; K cells express Fcγ R111. Using these effector cell populations and the target cells mentioned above, we have compared anti-NIP antibodies with different heavy chain constant domains for their ability to induce ADCC through human Fcγ R1, Fcγ R11 and Fcγ R111. The results suggest that all three human Fcγ receptors appear to recognise a binding site on IgG within the lower hinge (residues 234–237) and trigger ADCC via this site, but that each receptor sees this common site in a different way. The possibility that other amino acid residues also participate in the binding/triggering site(s) cannot be excluded.