We have developed a sensitive and flexible method for the qualitative evaluation of IgG-Fc receptor interactions in cell suspensions. The assay is based on the flow cytometric quantitation of antibody-coated erythrocyte (EA) rosetting using fluorescein-labelled indicator erythrocytes (E). The number of IgG molecules on indicator E, an important parameter in EA rosetting, was estimated by calibrated flow cytometry. EA binding quantitated by this method was correlated with microscopically evaluated rosette formation. Besides automated quantitation of EA binding, this method offers the additional advantage of simultaneously using a second fluorescence parameter, permitting analysis of FcR activity in subpopulations of cells. As an example of the applicability of this approach the binding characteristics of E sensitized with a series of murine heavy chain isotype switch variant monoclonal antibodies against glycophorin A, to the low affinity receptor on K562 cells were determined. Remarkably, the results suggest a comparable affinity of Fc gamma RII on these cells for immunoglobulins of the murine IgG1, IgG2a and IgG2b isotypes.