Monocytopoietic proliferation activity was investigated in patients with untreated Hodgkin's disease, Hodgkin's disease in long-term complete remission, and untreated non-Hodgkin's lymphoma of the lymphosarcoma and reticulosarcoma type. Untreated Hodgkin's disease was found to be associated with a rise in medullary monocyte production which returned to normal during long-term complete remissions. In contrast, monocyte production was increased in only 5 out of 14 patients with lymphosarcoma and reticulum cell sarcoma, normal in 3, and reduced in 6. In neither of these lymphomas was any relation between monocyte production and stage or histology of the disease detectable.