Peripheral blood mononuclear cells from 23 patients with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML) or aplastic anemia (AA) undergoing allogeneic bone marrow transplantation were examined for spontaneous and Concanavalin A (Con-A)-induced suppressor activities against proliferative response of autologous or allogeneic responder lymphocytes to phytohemagglutinin (PHA) and for its relationship with acute or chronic graft-versus host disease (GVHD). Patients who developed acute GVHD had increased spontaneous and deficient Con A-induced suppressor cell activity, whereas patients who developed chronic GVHD or had no evidence of GVHD had both spontaneous and Con A-induced suppressor cell activity essentially similar to that of healthy controls. The nature of underlying disease (ALL v. AML v. AA) had no relationship with the abnormal suppressor functions. Depletion of adherent monocytes from mononuclear cells from a patient with acute GVHD and deficient Con A-induced suppressor function, restored the suppressor function, demonstrating the role of monocytes for deficient Con A suppressor function in some patients with acute GVHD. Additional mechanisms of abnormal immunoregulatory functions in bone marrow transplant recipients are discussed.