More than 95% of patients with chronic myelogenous leukemia (CML) contain an abnormal chromosome termed the Philadelphia chromosome (Ph1). Ph1 and the resulting BCR-ABL fused genes are markers for this type of leukemia. The product of the fused BCR-ABL genes is a protein of about 2000 amino acids termed P210 BCR-ABL. Although the BCR-ABL protein can be routinely detected in blood cells from blast crisis CML patients by assaying for its activated tyrosine kinase activity, detection of P210 BCR-ABL in early stage CML patients (chronic phase) has not yet been possible (S. A. Maxwell et al., Cancer Res., 47: 1731, 1987). A procedure involving Western blotting with an anti-ABL monoclonal antibody was developed that allows detection of P210 BCR-ABL and P145 ABL in cells from chronic phase and blast crisis CML patients, but as expected only P145 ABL was found in normal white blood cells. Most chronic phase patients also contained one to two ABL proteins with a molecular weight of about 190,000. Interestingly, the ratio of BCR-ABL to ABL proteins increased in four blast crisis patients compared to 18 chronic phase patients. Also, one chronic phase patient analyzed on three separate occasions lacked P210 BCR-ABL and exhibited only the Mr 190,000 form. This assay should also be useful in other leukemias that express altered forms of the ABL protein.