The ability of cytoimmunologic monitoring to detect acute rejection in heart and heart-lung transplant recipients was assessed. Seventeen heart and nine heart-lung transplant patients treated with cyclosporine as the primary immunosuppressive agent were monitored after operation. Seventeen acute rejection episodes occurred in the heart-only recipients and 12 in the combined heart-lung recipients. These acute rejection reactions were accompanied by increased numbers of 'activated' lymphocytes in all but one case, providing cytoimmunologic monitoring a sensitivity of 100% in the heart-lung recipients and 94% in the heart recipients. The specificity was, however, low (68% in the heart recipients and 55% in the heart-lung recipients). Seventy-two percent of the false-positive results were associated with viral infections, particularly cytomegalovirus infections.