Tissue specimens obtained at autopsy from seven childhood cases of malignant histiocytosis were studied by immunohistochemistry. Clinically, the majority of the cases showed sustained fever, hepatosplenomegaly, pancytopenia, and DIC. The pretreatment diagnosis was based on their typical clinical manifestations and bone marrow smear findings. Although three patients temporarily responded to exchange transfusion and chemotherapy, all seven patients eventually died of active disease. Postmortem examination revealed the proliferation of atypical histiocytes appearing in variable degrees of maturation in the lymph nodes, liver, spleen, bone marrow, lungs, and central nervous system. Immunohistochemical staining for lysozyme, nonspecific cross-reacting antigen (NCA), alpha 1-antitrypsin (alpha 1 AT), alpha and beta subunits of S100 protein (S100 alpha, beta), and concanavalin A receptors (ConAR) in cytoplasm demonstrated the presence of two subtypes of malignant histiocytes, ie, S100 beta+/NCA-/ConAR+ (4 cases) and S100 beta-/NCA+/ConA R+ (three cases). The results of lysozyme, alpha 1 AT, and S100 alpha staining were inconsistent. A survey of the literature disclosed that the incidence of S100 protein-positive cases in children was higher than in adults (12/21 v 5/19; chi 2, P less than .05). Further large scale investigation is necessary to confirm the independence and significance of these two subtypes of histiocytes in malignant histiocytosis.