The T-lymphocyte subset interactions in the cell-mediated response to Epstein-Barr virus (EBV) were studied in an in vitro system in which the ability of T lymphocytes to inhibit outgrowth of autologous EBV-transformed B lymphocytes is assessed. Inhibition could be demonstrated within lymphocytes of both the T4 and T8 surface phenotypes. Outgrowth inhibition was observed more frequently when the effector T-cell population contained cells of both surface phenotypes. In blocking experiments the OKT3 antibody completely prevents development of outgrowth inhibitory activity; the OKT4 and OKT8 antibodies were less effective in interfering with outgrowth inhibitory function. Maximal blocking activity occurred when antibody addition occurred in the early phase of generation of suppressor function. Pharmacologically achievable concentrations of interferon-alpha restored outgrowth inhibitory activity after blocking with monoclonal antibody. EBV reactivation was easily demonstrable in a group of 10 renal allograft recipients who received OKT3 antibody for treatment of acute rejection. These studies suggest that the immunoregulatory control of proliferation of EBV-transformed B lymphocytes is complex, and involves a collaborative interaction of lymphocytes of both the T4 and T8 surface phenotypes.