Six children with histiocytosis X, all in clinical remission, were investigated for antibody-dependent cytotoxicity mediated by monocytes and neutrophils. Monocytes demonstrated a reduced cytotoxicity and a normal Fc receptor activity. ludged by light microscopy the monocytes were normal. Preincubation with the patient's serum did not influence the cytotoxicity of normal monocytes. Neutrophils from patients with histiocytosis X showed a normal cytotoxic activity. We postulate a functional defect of the mononuclear phagocyte system in histiocytosis X.