The rearrangement of breakpoint cluster region (ber) was examined in leukemic cells obtained from 3 patients initially diagnosed as having Ph+ acute leukemia, 2 with acute lymphocytic leukemia (ALL) and one with acute mixed leukemia. DNA was digested with Bgl II and BamH I. The ber rearrangement was present in the case of acute mixed leukemia (Case 1), but was absent in the 2 cases of ALL (Cases 2 and 3). These results suggest that Case 1 represented a type of blast crisis of chronic myelocytic leukemia which was unusual in the sense of the occurrence of a myeloid-lymphoid conversion and lack of an apparent chronic phase. Cases 2 and 3 appeared to be de novo Ph+ ALL.