Peripheral blood mononuclear cells from 14 patients with acute leukemia or aplastic anemia undergoing allogeneic bone marrow transplantation were examined for the autologous mixed lymphocyte reaction (AMLR) between T and non-T cells and its relationship with chronic graft-versus-host disease (GVHD). Five of 6 patients with GVHD demonstrated deficiency of the AMLR, whereas only three of 8 patients with no evidence of chronic GVHD had deficient AMLR. The nature of underlying disease had no effect on the AMLR. The significance of these results is discussed.