We investigated the expression of CD56 (a neural cell adhesion molecule, NCAM) and CD57 in various hematopoietic and non-hematopoietic malignant cells, using Leu-19 and Leu-7 monoclonal antibodies. Although both molecules are commonly defined as a natural killer cell marker, we found that CD56 was highly expressed on blasts from patients with acute monocytic (4/6) and megakaryocytic (3/3) leukemias. In the latter, FACS two-color analysis revealed that leukemic megakaryoblasts simultaneously expressed CD56 and platelet-related antigens. Among leukemic cell lines, one myelocytic, three monocytic, and two megakaryocytic lines were positive for CD56. On the other hand, except for one large granular lymphocytic leukemia and one multiple myeloma cell line, none of the lymphoid leukemia cell lines or lymphoblasts from patients with acute lymphocytic leukemia (ALL) (0/15), non-Hodgkin's lymphoma (NHL) (0/2), and central nervous system (CNS) leukemia (0/2) reacted with Leu-19 antibody for CD56. The expression of CD56 in leukemia cells was not significantly affected by 12-O-tetradecanoylphorbol-13-acetate (TPA). By contrast, all hematopoietic materials were negative for CD57, while non-hematopoietic neuroblastoma cell lines expressed this molecule (4/5) as well as CD56 (5/5). Cytogenetically, the NCAM gene is located at chromosome 11q23, and chromosome breaks were often observed at this location in various leukemias. Blasts from all five acute non-lymphocytic leukemia (ANLL) patients and cell lines with 11q23-proximal chromosomal breaks were positive, while those from one ALL patient with an 11q23 abnormality were negative for CD56, necessitating further studies to clarify the link between the 11q23 abnormality and CD56 expression.