Terminal blast crisis cells of chronic granulocytic leukemia are biochemically distinct. Triton-X-114 detergent phase lysates revealed that myeloid types express predominantly proteins of a 24 kd apparent molecular weight range, whereas lymphatic types do not express these molecules but a 55 kd protein band. These biochemical differences, observed in the poorly differentiated blast crisis cells, are also found in well differentiated hemopoietic malignancies such as chronic lymphatic leukemia, or in mature granulocytes isolated from healthy individuals. The results support the concept of the different lineages of blast crisis cells in chronic granulocytic leukemia but question the role of these proteins in cell differentiation. In addition, the presence or absence of these proteins provide a helpful tool for classifying blast crises of chronic granulocytic leukemia.