Blast cells from a 39-year-old man in the blastic phase of chronic myeloid leukemia, with a benign phase of 15 years duration, as well as a cell line arising from this cell population, were studied. Cellular morphology, cytochemical staining pattern, and absence of terminal deoxynucleotidyl transferase showed the blast cells to be of myeloid character. Cytogenetic studies revealed the presence of two near-haploid cell populations with +8 and +8, +15, respectively, both of them containing the translocation t(9;22) in the original tumor cell sample. The cell line derived from this patient's leukemic cell sample contained both near-haploid and hyperdiploid clones, the hyperdiploid clones being multiples of the near-haploid clone(s). All of the clones carried the t(9;22) in the form of a Philadelphia chromosome.