The regulation of Ia (HLA-DR) antigen expression on myeloid progenitor cells may be closely related to the control of myelopoiesis in both normal individuals and chronic myeloid leukemia (CML) patients. In an effort to study directly the expression and behavior of Ia surface molecules on myeloid progenitor cells, we used an immunologic purification technique to enrich these cells approximately 100-fold from the peripheral blood of CML patients. The majority of cells in this blast population expressed HLA-DR antigens. Thirty percent to 40% of cells could form a granulocyte or monocyte colony in agar, and these cells tended to express the highest levels of HLA-DR. The number of HLA-DR molecules per cell increased about twofold as the cells tranversed the cell cycle from G0/G1 to G2/M. This was true for unstimulated cells or cells exposed to colony-stimulating factors. Some of this increase was related to a corresponding increase in cell size and is also seen with other cell surface antigens such as beta-2-microglobulin. Ia antigen expression was not modified by culture with colony-stimulating factors, fetal calf serum, or serum-free, prostaglandin-free medium for periods of up to 24 hours. These results demonstrate that Ia antigens are expressed on the myeloid progenitor cells of CML, are increased in the S and G2/M phases of the cell cycle, and are stable under most in vitro culture conditions for at least 24 hours of culture.