Leukemic cells from 15 patients with acute myelomonocytic leukemia (M4) were investigated by light and electron microscopy. As controls, the blood cells from 2 normal volunteers and 3 acute monocytic leukemia (M5b) were examined. To identify monocytic cells, a double esterase (DE) staining combining with alpha-naphthyl butyrate esterase (ANBE) and naphthol-ASD chloroacetate esterase (N-ASD-CAE) stainings was used. According to DE staining, M4 leukemic cells were classified into ANBE positive cells, N-ASD-CAE positive cells and DE-positive cells coated vesides. Ultrastructurally, M4 leukemic cells possessed indented nuclear membrane, clusters of myeloperoxidase-positive rod-like granules, coated vesicles, abundant smooth endoplasmic vesicles and invaginated cell membrane similarly to M5b leukemic cells and normal monocytes. These features were observed not only in the ANBE positive cells but also in the DE-positive cells. The N-ASD-CAE positive cells in M4 ultrastructurally showed atypical features of polymorphonuclear leukocytes and possessed monocytic characteristics to some extents. M4 leukemic cells occasionally showed some peculiar structures such as the reticulated lamellae complex, paired cisternae and annulate lamellae. The cells showing erythrophagocytosis were either observed. Combining the results of DE staining and electron microscopy, it was suggested that M4 is closely associated with the malignant transformation of a monocyte/granulocyte precursor cell which has an ability to differentiate to both monocyte and granulocyte.