Red cells coated with IgG globulin were bound firmly to human mononuclear cells and formed rosettes. Rosette formation occurred when red cells were coated with IgG attached either immunologically (anti-D, anti-penicillin, or Donath-Landsteiner antibodies) or nonimmunologically with chromic chloride; no attachment was observed with cells coated with albumin. Rosette formation was blocked by pretreatment of white cells with sulfhydryl-binding reagents. Metabolic inhibitors did not prevent red cell adherence. White cells of other primates demonstrated a high degree of species specificity. Ultrastructural studies showed that the predominant leukocytes involved in rosette formation were monocytes, but some cells with characteristics of lymphocytes also formed rosettes. Considerable interdigitation of cell surfaces occurred at attachment sites and bound red cells appeared deformed. Thus, these studies confirm the presence of specific surface receptors for IgG on human monocytes and suggest that such receptors may provide a mechanism by which large numbers of red cells are eventually destroyed.