A case is described of acute leukemia whose neoplastic cells possessed myeloid and lymphoid characteristics. Neoplastic cells possessed cytoplasmic granules containing Sudan black B material and diaminobenzidine myeloperoxidase. In addition, these luekemic cells were positive for terminal deoxynucleotidyl transferase. Ia antigen, and the common acute lymphocytic leukemia antigen. These findings indicate that biphenotypic cell markers may exist in cases of acute leukemia. It is our belief that these results are best explained as either a mixed myeloid-lymphoid leukemia or a stem cell leukemia capable of differentiating into myeloid and lymphoid cells.