When phenotypic marker analysis of acute lymphoblastic leukemia (ALL) cells (102 cases) was performed, a group of ALL cells (15 cases) classified as non-T non-B, and negative for common-ALL antigen (CALLA) was characterized in a focused manner. “Non-T non-B” was defined as negative for T cell properties such as E-rosetting or reactivity with anti-human T-cell monoclonal antibodies (T101, WT1), and absence of any B-cell characteristics (cell surface and/or cytoplasmic immunoglobulin and reactivity with B1 monoclonal antibody). Despite their marked heterogeneity, CALLA(-) non-T non-B ALL cells revealed three different phenotypic patterns in terms of presence of terminal deoxynucleotidyl transferase (TdT) and of reactivity with antimyeloid (MCS1) or myelomonocyte (MCS2 and OKM1) monoclonal antibodies. Four of 15 cases reacted with some myeloid-specific antibodies, but were negative for TdT. Six cases had both MCS2 antigen and TdT. The remaining five cases expressed no myeloid antigens, but were positive for TdT with some exceptions. These findings showed that acute leukemias with myeloid antigens might be involved in CALLA(-) non-T non-B ALL having no relationship to the presence of TdT, and, furthermore, that the blasts with simultaneous expression of TdT and myeloid-specific antigen (MCS2) might represent an immature stage in hematopoietic differentiation closely corresponding with the bifurcation of the lymphocyte/myeloid pathway. Alternatively, only five cases remained “unclassified leukemia.” We therefore think that the detailed examination of CALLA(-) non-T non-B ALL cells using myeloid specific antibodies is helpful in clarifying the characteristics of myeloid precursors and the common bipotential stem cell of lymphoid and myeloid progenitors.