While immunotyping blast cells from 45 patients with CML blast crisis, we detected 5 cases with immunologically primitive blast cells. The immunological phenotype of these cells corresponded to that of primitive stem cells which are characterized by expression of CD34 and HLA-DR antigens in the absence of other immunological markers. We suggest that blast cells from these patients may undergo differentiation similar to that of primitive stem cells that implies the existence of a new immunological variant of CML blast crisis, a primitive variant. Morphologically, blast cells in 3 cases could be classified as myeloid, in 2 cases precise identification was impossible. Cytochemically, this type of cells can be defined as mixed. The patients with CD34+ phenotype do not differ clinically or hematologically from those with CML blast crisis. Blast cells with membrane marker CD34 are likely to arise in any CML phase either as a component of overall leukemic population or predominant, single subclone.