Recent observations of chimerism in patients relapsed following an allotransplant suggest the persistence of immunotolerance, thus offering a biologic rationale for the use of donor lymphocyte transfusion (DLT), In this study, we have analyzed by PCR amplification of several VNTR regions, sequential bone marrow and peripheral blood DNA samples in four patients who received DLT for CR IL relapse after bone marrow transplantation. Prior to DLT, all patients showed mixed chimerism in peripheral blood cells while two had mixed chimerism and two no chimerism in the PM, None of these four patients showed evidence of chimerism at the cytogenetic level (all had 100% +ve metaphases), After DLT, a complete hematologic and molecular remission (ie disappearance of the BCR/ABL fusion transcript) was obtained in the two patients who had bone marrow mixed chimerism prior to DLT, The two patients without evidence of marrow chimerism prior to DLT converted to a pattern of mixed chimerism after DLT, but both developed a severe bone marrow aplasia occurring at day 56 and 36, respectively, With regard to the sequential analysis of bone marrow chimerism after DLT we observed that: (1) the disappearance of BCR/ABL +ve cells paralleled the conversion to a pattern of full donor chimerism; and (2) the time interval to achieve CR was inversely correlated with the percentage of donor DNA in bone marrow, In conclusion, we have shown here that the assessment of bone marrow pre-DLT chimerism by PCR analysis might predict the response in patients with favorable characteristics, and also might identify patients at high risk of developing severe myelosuppression.