In Philadelphia chromosome (Ph1)-positive acute lymphoblastic leukemia (ALL), some cytogenetic studies have suggested clonal derivation from a multipotential stem cell. The role of the product of the chimeric gene, P190, is not, however, well understood. We examined the expression of P190-type bcr/abl in single hematopoietic colonies obtained at various clinical stages of a patient with Ph1-positive ALL, using the polymerase chain reaction (PCR). Seven out of 58 colonies examined expressed P190-type bcr/abl. Five out of seven colonies were granulocyte/macrophage (GM) colonies and two were erythroid colonies. The cell lineages of these colonies were confirmed by testing for the expressions of the myeloperoxidase (MPO) gene in the GM colonies and the beta-globin gene in the erythroid colonies. These results suggest transformation of multipotential stem cell in this patient and confirm that expression of the P190-type bcr/abl fusion gene permits stem cell differentiation leading to Ph1-positive ALL.