Blood lymphocyte functional capacity and serum immunoglobulins were studied in 40 patients with Hodgkin's disease (HD) admitted to Radiumhemmet, Stockholm, before treatment and in complete remission 2-56 months following termination of radiotherapy (total nodal irradiation [TNI]; n = 29) or chemotherapy (MOPP; n = 11). Lymphocyte studies included determination of total lymphocyte and T-cell counts and evaluation of spontaneous DNA synthesis during the first day of culture and mitogen-(concanavalin A, pokeweed mitogen) and antigen (purified protein derivative, PPD)-induced activation on the third day. Blood lymphocyte and T-cell counts decreased dramatically following TNI. A slow restitution was seen, but pretreatment levels were not reached even four years following therapy. The responses to ConA and PPD but not PWM were significantly reduced shortly after TNI. The mitogen response did not increase with time as did the PPD response. Lymphocyte counts and lymphocyte stimulation, which were severely depressed before treatment of patients in the chemotherapy group, remained unchanged 2-36 months after termination of therapy. A significant reduction of IgM levels was observed regardless of the mode of treatment. Splenectomy prevented the profound reduction of blood lymphocyte and T-cell counts following therapy but did not influence the other immunologic variables under study.