The reciprocal translocation between chromosome 9 and chromosome 22, as observed in chronic myeloid leukemia (CML) as well as in acute lymphoblastic leukemia (ALL), results in a 22q- chromosome, the so-called Philadelphia chromosome. The translocation event creates on the Philadelphia chromosome a fusion between two genes: bcr and abl. Depending on the localization of the breakpoint in the bcr gene different chimeric bcr-abl genes are generated, each encoding their own tumor-specific protein: e1-a2P190bcr-abl, b2-a2p210bcr-abl, or b3-a2P210bcr-abl. Especially in ALL, the presence of such a tumor-specific protein is highly associated with a poor prognosis. Detection of these proteins therefore has a strong clinical significance. In this study a polyclonal antiserum, termed BP-2, was raised against a synthetic peptide, corresponding to the tumor-specific 'fusion-point' epitope of the b3-a2P210bcr-abl protein. The specificity of BP-2 for the bcr-abl joining region in b3-a2P210bcr-abl is demonstrated by means of peptide inhibition studies in combination with immunoprecipitation. In addition we show the reactivity of BP-2 with bcr-abl proteins in leukemic cells of a Philadelphia-chromosome-positive ALL patient.