In myocarditis OKIal-positive cells are increased, which is compatible with B cell activation and intensified immunoglobulin synthesis. ConA-induced and spontaneous T suppressor cell activity assays showed no significant difference from age-matched controls. In the acute phase of endocarditis T cells, and particularly T helper cells, are diminished and OKIal-and DR-positive B cells are increased in number. ConA-induced T suppressor cell activity in the indicator systems of allogenic mixed lymphocyte reaction and mitogen stimulation (ConA, PHA) was significantly reduced, whereas no alteration was observed in the other indicator systems when patients were compared to all or the daily controls.