The clinical course of Ph+CML and their terminal blastic stage is described in the following case histories. During blastic phase, cell surface markers, terminal deoxynucleotidyl transferase (TdT) activity, and flow cytometric measurements were used or determination of the blast cell phenotype which was undifferentiated by morphological and cytochemical criteria. A high proportion of blast cells expressed Fc receptors for IgG, TdT activity was normal in both children and RNA measurements of single cells flow cytometry showed a high RNA content in the majority of blasts. These findings are compatible with the phenotype seen in the myelomonocytic leukemias. Flow cytometry appears to be a useful adjunct to immunological methods and TdT activity for the rapid characterization of CML in blastic phase.