Monocyte functions were studied in 16 patients with Hodgkin's disease, 11 untreated and five in unmaintained complete remission. Eleven untreated patients with non-Hodgkin's lymphomas and 21 healthy persons were used as controls. Monocytes were isolated from peripheral blood and enriched to greater than 90%. Lymphoma monocytes showed normal ability to lyse human RBC coated with anti-D IgG antibodies as evaluated by a 51Cr-release assay. The ability of monocytes to augment or suppress concanavalin A stimulation of lymphocytes purified to greater than 98% was studied by incubation of a number of lymphocytes with increasing amounts of purified monocytes. The incorporation of 14C-thymidine was potentiated by a factor of 10 in the presence of equal amounts of monocytes. There was no difference between monocytes from Hodgkin, non-Hodgkin or healthy controls to augment patients' autologous or normal lymphocytes. Patient monocytes also suppressed the response at the same monocyte-lymphocyte ratio as normal monocytes. Stimulation of patient lymphocytes without the addition of monocytes was usually lower than that of normal control lymphocytes. The difference between patient and control lymphocyte stimulation was preserved in the presence of monocytes. It is concluded that monocytes from patients with active Hodgkin's disease or non-Hodgkin's lymphoma have normal helper and suppressor effects on patient or normal lymphocytes stimulated by Con A and normal antibody-dependent cytotoxicity.