The in vitro polyclonal pokeweed mitogen (PWM)-induced activation of human B lymphocytes is enhanced by addition of autologous or allogeneic irradiated T cells. This model for B/T-cell cooperation may be used to define and describe the balance between T helper and T suppressor phenomena. The present study investigates the helper and suppressor capacities of mononuclear cells isolated from peripheral blood of infectious mononucleosis patients during the acute disease and the reconvalescence period. During the acute disease we found a functional lack of T helper capacity; furthermore, the T cells were able to suppress the PWM and T-cell-dependent B-cell proliferation of healthy donor cells. The suppression was non-cytotoxic; i.e. not due to destruction of the responder cells. This phenomenon of non-cytotoxic suppression was found for all seven patients studied and disappeared during the reconvalescence period, indicating that the T lymphocytosis seen in infectious mononucleosis includes an expansion of T suppressor cells.