Chronic myeloid leukemia (CML) is characterized by the presence of a 210-kD protein (P210bcr-abl) in the cytoplasm of leukemic cells, generated by the reciprocal translocation between chromosome 9 and chromosome 22. Due to this translocation, the abl oncogene is coupled to the bcr gene, forming a new determinant in this protein encoded by the bcr-abl joining region. In the joining region itself, either the bcr exon 2 is coupled to the abl exon 2 (b2-a2), or the bcr exon 3 is coupled to the abl exon 2 (b3-a2). Thus, these joining regions form by definition new tumor-specific determinants in the respective chimeric P210-bcr-abl molecules. This paper addresses the question as to whether these tumor-specific joining regions are exposed on the P210bcr-abl molecule in such a way that antibodies can be generated to detect these sites. To test this possibility a polyclonal antiserum, termed BP-1, was raised against a synthetic peptide representative for the b2-a2 joining region. The reactivity of BP-1 was analyzed in an ELISA system on various synthetic peptides. Peptide inhibition studies showed the presence of antibodies to different parts of the b2-a2 peptide in the polyvalent antiserum. The reactivity of BP-1 was then tested with native P210bcr-abl molecules in various CML cell lines (K562, LAMA-84, and BV173) using a protein kinase assay. In this context, the bcr-abl junctions were first analyzed at the DNA and RNA level. The present study indicates that BP-1 specifically recognizes the b2-a2 junction in native P210bcr-abl. Furthermore, BP-1 clearly discriminates between b2-a2 P210bcr-abl and b3-a2 P210bcr-abl. We conclude that the tumor-specific b2-a2 joining region is antigenically exposed on the native P210bcr-abl molecule.