A new quantitative fluorometric binding assay that uses fluoresceinated aggregated IgG is proposed for the study of Fc receptors. The method was compared with a radiolabeling binding assay on three well characterized murine cell lines (38C-13, EL4, and BW). The apparent association constant of the binding and the amount of aggregated IgG bound per cell at saturation were calculated. The fluorometric assay enables the detection of 5 × 10-10 M bound aggregated IgG. Inhibition studies with monomeric IgG, reduced and alkylated aggregated IgG, and aggregated F(ab′)2 fragments of IgG confirmed the specificity of the assay. Staphylococcal protein A inhibited the binding of the aggregated IgG to Fc receptors.