The morphological and immunological features of 22 patients with transformation of Philadelphia chromosome (Ph1) positive chronic myelogenous leukemia (CML) were evaluated. Patterns of differentiation in blast crisis were as follows: myeloblastic 5 cases, lymphoblastic 6 cases, basophil differentiation 4 cases, monoblastic 3 cases, megakaryoblastic 1 case, and mixed 3 cases (myeloblastic and lymphoblastic, monoblastic and lymphoblastic and panmyelosis). Three cases were diagnosed as acute lymphoblastic leukemia (ALL) in early stage. After complete remission was achieved rapidly with vincristine and prednisolone, the hematological findings showed CML features. The immunological phenotype of lymphoblasts in lymphoid crisis was Ia+, CALLA+, surface and intracytoplasmic immunoglobulin negative. It was suggested that the neoplastic cells in two cases had B cell differentiation because of B1 or B4 positive. The terminal deoxynucleodidyl transferase (TdT) activity was examined by immunoperoxidase and immunofluorescence techniques on 11 cases. All TdT+ cells were the small lymphoid cells in 5 cases of lymphoid crisis and one case in mixed type. Heterogeneity of the terminal phase of CML was recognized. It was thus suggested that blast crisis of CML occurred in the pluripotent stem cell.