High concentration of cytochalasin B (CB) and vinblastine (Vb) inhibited the erythrocyte-antibody (EA) rosette formation of rat peritoneal macrophages (PM). Low concentration of Vb enhanced rosette formation. CB and Vb had the same effect on the rosette formation when anti-SRBC immunoglobulin (Ig) had been previously bound in vivo or in vitro to the Fc receptors (FcR) of the macrophages, suggesting that the intact function of the cytoskeleton is required for the last step of rosette formation. CB and Vb exert a reversible effect on FcR activity. Regeneration of FcR activity was observed also in the presence of 10 microgram/ml cycloheximide. The effect of CB and Vb on rosette formation depends both on the (sub)classes of antibody involved and on the nature of the particulate antigen. Rosettes with living trypanosomes displayed an extremely high CB sensitivity.