A new cell line designated JA-CML was derived from the peripheral blood of a patient with blastic phase CML. Sequential evolution of phenotypic and genetic markers was demonstrated during adaptation from primary to continuous culture in vitro. In the primary sample the majority of blast cells displayed the early T-cell markers, CD7, HLA-DR, and TdT, but were negative for the common ALL antigen (CALLA), CD4 and CD8. Simultaneously, unstimulated metaphase cells showed great karyotypic variation with a range of 43-46 chromosomes per cell. Clonal changes included the Ph chromosome t(9;22), loss of the Y and gain of several altered chromosomes. The cells grew slowly in suspension during the first 10 weeks of culture. During that time, cells still expressed the CD7 and HLA-DR antigens. Karyotypic analysis at ten weeks showed a pattern of 46,X,-Y,t(9;22),+8 in more than 90% of metaphases with disappearance of all other abnormal chromosomes noted in the original sample. A tetraploid subline exhibiting duplication of most chromosomes, including the Ph, comprised the remaining metaphases. Upon further cultivation in vitro, the cells transformed spontaneously over a period of several weeks, from T-lymphoid into myeloid cells. Expression of CD7 was lost, but reactivities with monoclonal antibodies to CD34, CD33 and CD13 were newly acquired. The karyotype was hypertriploid and all cells carried two copies of t(9;22) and lacked normal copies of No. 9 or Y. The cells have since maintained stable cytogenetic and phenotypic profiles. Molecular rearrangement of the breakpoint cluster region was identified in the primary blasts and the established line and T-cell receptor gene rearrangements were not found. These observations suggest that the leukemic blast arose from primitive stem cells, not irreversibly committed T cells, and that these stem cells retained the capacity to differentiate along the myeloid pathway.