Immune status was measured in 47 previously untreated patients with Hodgkin's disease. For all patients, a 5-year follow-up was established. Immunologic capacity was measured by delayed-type hypersensitivity tests to common recall-antigens; enumeration of T- and B-lymphocytes in the peripheral blood; in vitro lymphocyte responsiveness to mitogens, antigens, and allogeneic lymphocytes; and serum levels of immunoglobulins. Compared with healthy controls, skin reactivity was decreased in the patients (P less than 0.05), but was not a prognostic marker with regard to survival. Total lymphocyte counts and the numbers of T- and B-cells did not differ between surviving and deceased patients. Decreased in vitro lymphocyte responsiveness to phytohemagglutinin and impaired responding capacity of patients lymphocytes in the mixed lymphocyte culture (MLC), used as markers, were a poor prognostic sign (P less than 0.001). The relevant clinical parameters with regard to 5-year survival were age, stage, and B symptomatology (P less than 0.005). The prognostic information supplied by age plus responding capacity in the MLC exceeded the predictive value of any combination of clinical parameters. Lymphocyte stimulation to pokeweed mitogen and antigens, stimulatory capacity of patients' lymphocytes in mixed lymphocyte cultures, the spontaneous DNA-synthesis, and immunoglobulin levels, did not provide prognostic information. The use of mixed lymphocyte cultures in staging patients with Hodgkin's disease may refine the prognostic information supplied by age, stage, and symptomatology.