BCR-ABL fusion proteins show increased signaling through their ABL tyrosine kinase domain, which can be blocked by specific inhibitors, thereby providing effective treatment. This makes detection ofBCR-ABLaberrations of utmost importance for diagnosis, classification and treatment of leukemia patients.BCR-ABLaberrations are currently detected by karyotyping, fluorescencein situhybridization (FISH) or PCR techniques, which are time consuming and require specialized facilities. We developed a simple flow cytometric immunobead assay for detection of BCR-ABL fusion proteins in cell lysates, using a bead-bound anti-BCR catching antibody and a fluorochrome-conjugated anti-ABL detection antibody. We noticed protein stability problems in lysates caused by proteases from mature myeloid cells. This problem could largely be solved by adding protease inhibitors in several steps of the immunobead assay. Testing of 145 patient samples showed fully concordant results between the BCR-ABL immunobead assay and reverse transcriptase PCR of fusion gene transcripts. Dilution experiments with BCR-ABL positive cell lines revealed sensitivities of at least 1%. We conclude that the BCR-ABL immunobead assay detects all types of BCR-ABL proteins in leukemic cells with high specificity and sensitivity. The assay does not need specialized laboratory facilities other than a flow cytometer, provides results within ˜4 h, and can be run in parallel to routine immunophenotyping.