Two cases of leukemic malignant histiocytosis had similar morphologic and enzyme histochemical findings. Large blasts with low nuclear/cytoplasmic ratios, occasional azurophilic granules, and immature nuclei with nucleoli were seen in peripheral blood and bone marrow smears. Case 1 had occasional erythrophagocytosis, while in Case 2 it was rare. They were peroxidase negative, and very strongly positive by alpha-naphthyl butyrate esterase stain, the latter being inhibited by sodium fluoride. Acid phosphatase stains were also very strongly positive and were inhibited with tartaric acid. They were also stained granularly with PAS. Surface marker analysis revealed myeloid surface antigens, CD11+, CD13+ and HLA-DR+ in Case 1, and CD11+, CD13+, CD33+ and HLA-DR+ in Case 2. Immunoperoxidase stains of bone marrow biopsies revealed that lysozyme was positive in both cases. S-100 protein was strongly positive in Case 1, but weakly so in the skin tumor and negative in the bone marrow of Case 2. Electron microscopy showed both cases to be myeloperoxidase negative and rich in cytoplasmic organelles, such as lysosomes, mitochondria, and endoplasmic reticuli. Nuclei were irregularly shaped and nucleoli were present in virtually all the cells. These findings suggest that the malignant histiocytes in these two cases derive from bone marrow macrophages, and S-100 protein can also be detected in monocyte-macrophage derived histiocytes.