Studies on cell-membrane-bound proteins in the human hematopoetic system revealed that the expression of certain peptides is restricted to the differentiation lineage. We applied discontinuous polyacrylamide gel electrophoresis of triton X-114 lysates to identify such proteins for a new diagnostic approach in human leukemia. A polypeptide with an apparent molecular mass range of 24 kd (p24) was found predominantly in cells of chronic granulocytic leukemia (CGL), myeloic type of blast crisis, and normal granulocytes. The data presented here suggest a role of this protein in the biology of malignant cells in chronic granulocytic leukemia throughout the course of the disease.