Anti-leukaemia activity after allogeneic bone marrow transplantation has been studied extensively but its antigen specificity and effector cell phenotype remain unknown. Here we report a study in three recipients of autologous bone marrow transplantation done as part of the treatment for acute leukaemia, in whom we were able to detect innate specific anti-leukaemia activity post-transplant. One patient maintained selective cell-mediated cytolytic activity against her autologous leukaemic cells in the absence of cytolysis of her normal bone marrow mononuclear cells (BMMC). She remains in complete remission 3 years after ABMT for acute myeloid leukaemia (M5). A second patient was transplanted for acute lymphoblastic leukaemia and had detectable anti-leukaemia activity up to 20 weeks post-ABMT. At this point anti-leukaemia activity could no longer be demonstrated and the patient suffered a relapse 2 weeks later. A third patient was transplanted for AML (M4 Eo) and lacked detectable leukaemia-specific immune reactivity at 1, 3 and 6 months post-ABMT. She relapsed 6 months after her ABMT and returned to complete remission after further chemotherapy. She commenced treatment with alpha interferon and regained NK function. Furthermore, she developed high level cytolytic activity against her autologous leukaemic cells in the absence of activity against her remission bone marrow samples. She remains in complete remission 17 months after her initial relapse. This is the first report of an apparent association between in vitro leukaemia-specific cytolytic activity in individual patients after ABMT and clinical outcome. It encourages the theory that autologous immunomodulation may be useful in the future treatment of leukaemia.