The prevalence of complement dependent, cold-reactive lymphocytotoxic serum factors (LT) was studied in 80 untreated patients with Hodgkin's disease by a microcytotoxicity assay. Sera from 24 patients (30%) contained LT as judged from at least 50% lysis of lymphocytes from 16 of 23 randomly selected normal donors. The spontaneous incorporation of "C-thymidine into blood lymphocytes from patients with LT was significantly higher than that of lymphocytes from LT-negative patients and from healthy controls. Total lymphocyte and T-cell counts, Concanavalin A, and pokeweed mitogen-induced lymphocyte DNA synthesis were lower in patients with LT. Lymphocytotoxic sera were more frequently encountered in patients with B symptoms, advanced disease, or nodular sclerosis/lymphocyte predominance histopathologic characteristics. LT were often found in patients with large and tumor-involved spleens. The ability of patient's serum to inhibit control lymphocyte response to Concanavalin A stimulation did not differ between LT-positive and LT-negative patients. We conclude that the presence of LT is associated with a quantitative and qualitative impairment of blood T-lymphocytes in untreated patients with Hodgkin's disease.