In vitro lymphocyte response to phytohemagglutinin (PHA), concavalin A (Con A), and pokeweed mitogen (PW) was evaluated in untreated and treated patients with Hodgkin’s disease (HD). The responding capacity to PHA was depressed, though not constantly, in the untreated patients compared with the response of lymphocytes from normal individuals. The depression was more evident, at group level, when the cells were stimulated with suboptimal concentrations of PHA. Radiotherapy constantly induced a strong decrease or a complete loss of the responding capacity of the cells which persisted at low levels for many months. Some years after the initial course of treatment, the response was clearly depressed, but it was better in patients in remission than during relapse. Splenectomy did not affect the responding capacity of the cells. The depressive effect induced by chemotherapy was apparently less marked and persistent than that of radiation. Con A-and PW-induced lymphocyte transformation usually paralleled the PHA-induced response. The depressed response to PHA was not due to an inhibitory activity of HD serum. Washed HD lymphocytes in fetal calf serum were not stimulated better than in autologous plasma, nor were HD sera able to depress the response of normal lymphocytes to PHA, Con A, PW, and PPD. Supernatants from HD lymphocytes cultured for 24 h without any stimulant, and extracts of these cells were also unable to affect the response of normal lymphocytes to PHA.