The monoclonal antibody (MoAb) Bsp-1 was used to purify basophilic cells from leukemic blood of five patients with Philadelphia chromosome (Ph') positive chronic myeloid leukemia (CML) and two patients with acute myeloid leukemia (AML) characterized by the chromosomal translocation t(6;9)(p23;q34). When cultured, Bsp-1 positive cells from all CML and AML patients showed the same clonal karyotype changes observed in diagnostic buffy coat preparations, indicating that the basophilic cells were of leukemic origin. In contrast, T lymphocytes from four of five CML patients cultured in the presence of interleukin-2 (IL-2) showed a normal karyotype and were therefore not derived from the leukemic clone. Bsp-1 staining correlated with toluidine blue-positive basophils in chronic phase CML and with toluidine blue-negative blast cells expressing an immature myeloid phenotype in blast crisis CML and AML. Chromosome in situ hybridization showed that the ABL oncogene was translocated from chromosome 9 to chromosome 22 in the CML patients but remained on chromosome 9 in the AML patients. These results indicate that the breakpoint at 9q34 in CML is 5' of ABL, whereas the breakpoint at 9q34 in AML is 3' of ABL. Field inversion gel electrophoresis showed that the 9q34 breakpoint was not within 200 kb 3' of ABL in one of the AML patients, nor was there any rearrangement of the PIM oncogene locus at 6p21.