Flow cytometric intracellular myeloperoxidase (MPO) staining of leukemic blasts is a useful tool in diagnosis of leukemia subtype. Interpretation of high MPO-positivity can be a diagnostic challenge in B-lineage acute lymphoblastic leukemia (B-ALL). While very few such cases have been reported, high MPO positive B-ALL cases without additional myeloid antigen positivity are suspect and require further investigation. Three pediatric cases of B-ALL with strong MPO staining (clone 8E6; Invitrogen) at diagnosis and three others with negative MPO staining were studied by flow cytometry and immunohistochemistry. In-vitro drug cytotoxicity, oxidative stress generation, and immunophenotyping using other MPO clones were performed to further investigate MPO presence. Expectedly, normal myeloid cells in all six samples were positive and mature lymphocytes negative for MPO staining. However, MPO monoclonal antibody (mAb) obtained from clones other than Invitrogen and other myeloid-specific mAbs gave negative results suggesting false positivity of the initial MPO staining. Immunohistochemistry for MPO was also negative on all six cases tested. Furthermore, in-vitro vincristine cytotoxicity was greater in leukemic cells from MPO false-positive cases compared with MPO-negative B-ALL samples, demonstrating indirect lack of MPO activity. Moreover, drug treatment did not lead to generation of reactive oxidative species, also reflective of lack of significant MPO presence. The cause of false-positive MPO staining remains unknown in these three cases; a cross reactivity could be the culprit. Caution should be given to similar phenomena and detailed investigation may contribute to the understanding of altered protein expression in such outlier cases. © 2017 International Clinical Cytometry Society.