Philadelphia (Ph′)-positive acute lymphoblastic leukemia (ALL) is highly associated with two forms of chimeric bcr-abl proteins: P190bcr- abl and P210bcr-abl. Whereas P210bcr-abl also occurs in chronic myeloid leukemia, P190bcr-abl is uniquely expressed in Ph′-positive ALL. As a consequence, P190bcr-abl is preeminently a tumor-specific marker in leukemic cells of ALL patients. Because P190bcr-abl is composed of the normal bcr and abl proteins, the major part of the P190bcr-abl molecule comprises nontumor-specific determinants. The joining region between bcr and abl, newly generated during the Ph′ translocation, is exclusively a tumor-specific epitope on the P190bcr-abl molecule. Therefore, only antibodies against the bcr-abl joining region will detect the tumor-specificity of P190bcr-abl. In this study a polyclonal antiserum, termed BP-ALL, was raised against a synthetic peptide corresponding to the bcr-abl junction in P190bcr-abl. The reactivity of BP-ALL with native P190bcr-abl derived from a Ph′-positive ALL cell line (TOM-1) was tested using immunoprecipitation analysis. BP-ALL reacted highly specifically with P190bcr-abl but not with P210bcr-abl isolated from chronic myeloid leukemia cell lines. Peptide inhibition studies further confirmed the fine specificity of BP-ALL. Our data indicate that the tumor-specific bcr-abl junction domain is exposed in an antigenic fashion on the P190bcr-abl molecule.