The sensitivity of the conventional EA (Fc) rosetting assay for detecting Fc receptors could be significantly increased by coating sheep erythrocytes (SRBC) with high concentrations of a preparation of homogeneous monoclonal IgG2b anti-SRBC antibodies. This was made possible by virtue of the fact that the monoclonal antibody preparation used was completely incapable of directly hemagglutinating SRBC, although it possessed a high titer as assessed by direct hemolysis or direct hemagglutination. The monoclonal IgG2b- coated SRBC indicator cells were compared to SRBC coated with subhemagglutinating amounts of the IgG fraction of a hemagglutinating rabbit anti-SRBC antiserum: at high coupling concentrations, the monoclonal antibody-coupled SRBC gave consistently higher percentages of Fc rosettes, a large proportion of which were large and multi-layered, when various lymphoid cell populations and a variety of Fc receptor-positive cultured tumor cell line populations were tested. The increased sensitivity of the Fc rosette attained by this method may depend on the nature of the monoclonal anti-SRBC reagent used. Thus, indicator cells prepared by incubation with high concentrations of IgG1 subclass-specific non-hemagglutinating monoclonal anti-SRBC antibodies always gave significantly lower percentages of Fc rosettes than conventionally prepared indicator cells.