We are investigating the interactions of recombinant human IgG antibodies with Fc receptors to enable selection of a constant region giving minimal depletion of antigen-bearing cells. Eight variant constant regions were made by substituting motifs between human IgG subclasses in the lower hinge region and/or a specially close loop of the CH2 domain. Mutations in the lower hinge region were shown to eliminate FcγRI binding and monocyte activation [Eur. J. Immunol. 29 (1999) 2613]. Here, we detail interactions with FcγRIIa of the 131R and 131H allotypes and FcγRIIb. Lower hinge mutations caused large reductions in binding whereas modification of residues 327, 330 and 331 had less dramatic effects. However, like the wildtype IgG subclass binding hierarchies, the effect of the mutations varied between different receptors. We identified IgG1 variants which react with the activating receptor, FcγRIIa, at least 10-fold less efficiently than wildtype IgG1 but whose binding to the inhibitory receptor, FcγRIIb, is only four-fold reduced. Manipulation of interactions with FcγRIIb separately from those with activating receptors provides potential for designing antibodies with novel and effective combinations of attributes. In addition, insight is gained into the evolution of functional differences in human IgG subclasses.