Two types of EA rosettes were studied in rheumatoid arthritis patients and control subjects. Heterologous (rabbit) and homologous (anti-D) antibody sensitized human red blood cells (EAr and EAh, respectively) were used for examining the EA binding capacity of human lymphocytes. Both control and rheumatoid arthritis blood samples yielded significantly lower EArRFC levels as compared to the percentage of EArRFCs, the difference being even more expressed with control lymphocytes. No difference, however, was observed in the case of tonsillar lymphocytes, as equal rosette levels were reached in both systems used. Introduction of immunosuppressive therapy resulted in a temporary dissociation of the two rosette values EAr and EAh binding lymphocytes were differently affected by aggregated human IgG, only the latter ones being inhibited, contrasting with the equal inhibitory action of purified Fc fragments. Reducing the density of anti-D antibodies on the surface of erythrocytes, the EAh rosette count dropped to the level of EAr RFCs. The data suggest an activity-depending Fc receptor distribution resulting in different binding capacity for more or less sensitized indicator RBCs.