Blood lymphocytes from 9 patients with Hodgkin's disease (HD) were studied. The results were compared with those of 6 seminoma testis patients and 9 healthy unrelated controls. All patients were in complete and unmaintained remission more than 10 years after termination of radiotherapy. The mean T-lymphocyte count of HD patients was lower than that of controls and seminoma testis patients. Lymphocyte DNA synthesis induced by pokeweed mitogen and phytohaemagglutinin was normal in both patient groups. Concanavalin A-induced DNA synthesis was low in 4 patients with HD although the mean stimulation of the group did not differ from controls or seminoma testis patients. Lymphocyte activation by PPD was slightly decreased in the 2 patient groups. No increase in spontaneous lymphocyte DNA synthesis was observed. The responding and stimulatory capacity in mixed lymphocyte culture was decreased in 3 and 2 HD patients respectively. 4 out of the 9 patients with HD but none with seminoma testis displayed severe impairment in T-lymphocyte functions. As 1 of the 4 had been treated solely by surgery, late effects of irradiation can only partly explain the results. The results may favour a hypothesis postulating a constitutional defect contributing to the immunoincompetence in HD.