A 43-year-old woman with Ph1-positive chronic myelogenous leukemia (CML) was diagnosed as having blastic crisis. The phenotype of blasts was CD9+, CD10+, CD19+, CD11b+ and CD33+, suggesting the B Lymphoid and myeloid mixed lineage. Two color analysis of CD10 and CD33 revealed that 50% of blast cells had both B lymphocyte- and myelomonocyte-associated surface markers. Rearrangement of the immunoglobulin heavy chain gene was detected. After culturing blasts with 12-o-tetradecanoyl-phorbol 13 acetate (TPA), basophilic granules appeared in cytoplasm of the cells. These granules were positive for toluidine blue staining. This finding that the biphenotypic blasts expressing both B lymphoid and myelomonocytoid features differentiated into basophils suggests that blasts of this case are derived from a common progenitor of B lymphoid and myeloid lineages including basophil.