Leukemias showing a conspicuous lymphoid phenotype, ie, those that are HLA-DR positive, common acute lymphoblastic antigen (cALLA) positive, terminal deoxynucleotidyl transferase (TdT) negative, as well as myeloperoxidase positive (MPO), could be considered so-called mixed leukemias. Leukemias with biphenotypic blasts have to be distinguished from cases comprising two separate subpopulations that express different lineage-associated characteristics. By use of a simple new method (Immunogold Staining) we examined a case of chronic myelomonocytic leukemia in blastic phase and demonstrated simultaneous staining for MPO/alpha-naphthyl-esterase and expression of the HLA-DR- positive, cALLA-positive, and TdT-negative phenotype. The cALL antigen was detected only on mono- and myelo-monoblasts; its expression was inversely related to the MPO positivity, and it disappeared together with these types of blasts after chemotherapy. On the basis of our findings it remains obscure whether the cALL antigen at the initial presentation was due to the immature monocytic features of the leukemic cells or disclosed on additional lymphoid differentiation pattern of the blasts.