Peripheral blood mononuclear leukocytes (cells of marrow donor origin) from 89 patients were collected at various times after allogeneic marrow transplantation, stimulated in vitro by phytohemagglutinin, and assayed for the production of interleukin 2 (IL-2). This was done by testing culture supernatants for their ability to induce proliferation of human lymphoblasts and/or IL-2-dependent cultured murine cytotoxic cells. Supernatants from cultures of patient cells were compared with those of marrow donor cells. Supernatants produced by cells from most short-term marrow recipients (30–101 days postgrafting), regardless of the presence or absence of acute graft-versus-host disease (GVHD), and those from most long-term patients with chronic GVHD (103–1932 days postgrafting) had significantly lower-than-normal IL-2 activity, whereas cells from most long-term marrow recipients without GVHD (353–1934 days postgrafting) had essentially normal IL-2 activity. Additionally, we tested the ability of monocytes from 35 marrow recipients to produce interleukin 1 (IL-1) in response to lipopolysaccharide as compared with monocytes from marrow donors or normal unrelated individuals. IL-1 activity in culture supernatants of patient cells, regardless of the time of testing after marrow grafting and the status of GVHD, was found not to differ from that in supernatants of normal cells. These findings suggest that impaired T cell functions seen in some (but not all) marrow recipients are probably not due to IL-1 but to IL-2 deficiency or to the mechanism that causes IL-2 deficiency.