Potential effect of the interaction of CD2 with its ligand LFA-3 on B cell responses in vitro was studied to evaluate the action of CD2-LFA-3 in immune responses. It was found that: 1) addition of autologous erythrocytes to unfractional mononuclear cells resulted in enhancement of PWM-induced lymphocyte proliferation, and had no significant influence on non-mitogen stimulation; 2) autologous erythrocytes potentiated the synthesis of immunoglobulins; and 3) costimulating effects of autologous RBC can be depressed by preincubation of PBMNC with anti-CD2 monoclonal antibody (WuTll). The results suggest that preincubation of mononuclear cells with anti-CD2 McAb has a down-regulating effects on B cell responses and the interaction of CD2 with its ligand LFA-3 expressed by RBC is closely related to immune responses.