Seventy-one previously untreated patients with non-Hodgkin lymphomas were studied with several readilyvailable tests of immune function: number of peripheral blood lymphocytes, serum immunoglobulins, and delayed hypersensitivity to six recall antigens. The results were correlated to histology (Rappaport classification), stage (Ann Arbor classification), the presence of symptoms, and survival. As a group, 38 patients with diffuse lymphomas exhibited marked impairment in reactivity to five of six antigens (p less than 0.03 to p less than 0.001). In addition, lymphopenia and reduced levels of serum IgA were found in association with diffuse histiocytic lymphoma. Among patients with diffuse lymphoma, lymphocyte number and skin test reactivity tended to be greater in those with localized disease or without constitutional symptoms, and survival was superior for patients free of symptoms (p less than 0.01). As a group, 33 patients with nodular lymphoma had normal numbers of lymphocytes, lower levels of serum IgG and IgA, and significant impairment of reactivity to two antigens (streptokinase-streptodornase and mumps; p less than 0.01); reactivity to three other antigens (Candida albicans, coccidiodin, and tuberculin) was normal. Survival for patients with nodular lymphoma was superior (p less than 0.01) compared to those with diffuse lymphomas. In summary, severe immunodeficiency was found in patients with diffuse lymphoma (particularly diffuse histiocytic lymphoma), and definite but much less severe immunodeficiency was characteristic of patients with nodular lymphoma.