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We used a modification of the polymerase chain reaction (PCR) to amplify the specific bcr-abl mRNA from 14 patients with chronic myeloid leukemia (CML) who had previously received non T cell depleted allogenic bone marrow transplantation (BMT). Two types of reactions were used: a single step amplification with 5' and 3' primers, and a double step PCR in which products of the first amplification were reamplified using nested primers. The latter procedure was highly sensitive and capable of detecting one abnormal cell in 10(7) cells. At the time of PCR analysis, all 14 patients were in hematological remission, and 13 were in cytogenetic remission. PCR analysis revealed rearranged bcr-abl mRNA in five patients. The interval from transplant in those five patients ranged from 3 to 63 months. Two of the five positive patients were reexamined after 3 months and were found negative by double step PCR. Our findings suggest that after non-T cell depleted BMT the eradication of the leukemic clone probably occurs in some patients. Other patients, however, proved to have a small number of abnormal cells even at long intervals after BMT, although these cells could only be detected transiently in some patients. The significance of these abnormal cells with respect to the risk of leukemic relapse remains to be determined.