Immune complexes were prepared by incubation of human IgG paraproteins with F(ab')2 fragments of the mAb K35 against the kappa-L chain of human IgG. The composition of these complexes was analyzed by centrifugation over sucrose gradients, by gel filtration, by RIA with either IgG Sepharose or K35 Sepharose and by double-labeling studies. The results indicated that the complexes consist of saturated tetramers composed of two IgG molecules cross-linked by two F(ab')2 fragments of the mAb. These complexes were used to study the binding of the different IgG subclasses to human neutrophils at 4 degrees C. Human neutrophils bound IgG3 complexes approximately three times faster than IgG1 complexes. Binding of IgG2 or IgG4 dimers to the neutrophils was undetectable. The same number of IgG1 complexes and IgG3 complexes bound to the neutrophils, but considerable inter-donor variation was found (mean number of Fc gamma R per neutrophil: 190,000, range 120,000 to 400,000). The Ka for the binding of IgG1 complexes to neutrophils (median 11 x 10(7) M-1) was lower than the Ka for the binding of IgG3 complexes (median 47 x 10(7) M-1). Competition studies between labeled IgG1 complexes or IgG3 complexes and unlabeled complexes showed that the Fc gamma R of human neutrophils do not display an IgG subclass specificity. Incubation of neutrophils with a mAb against the FcRIII completely blocked the binding of IgG1 complexes and IgG3 complexes. Incubation with a mAb against the FcRII reduced the affinity of the complexes for the neutrophils but had no effect on the maximum number of complexes bound. This indicates that one complex may bind simultaneously to one FcRIII and to one FcRII.