Patients with Hodgkin's disease, at presentation or in remission, exhibit a persistent defect in cellular immunity. Natural killer cell mediated cytotoxicity is depressed in untreated patients. Humoral immune function is transiently reduced following treatment. The cellular immune defect appears to be the result of enhanced sensitivity to suppressor monocytes and T-suppressor cells, in addition to abnormal interleukin-2 production. Patients with advanced disease have an inherent T-lymphocyte defect. Reed-Sternberg cells function as antigen-presenting cells for mitogen-induced and mixed lymphocyte T-cell proliferation.