ln an attempt to clarify the dual origin histiocytes and to reclassify histiocytic proliferative disorders according to their immunohistochemical properties, normal histiocytes and histiocytes in selected proliferative disorders were stained using the peroxidase-antiperoxidase method for lysozyme, α 1-antichymotrypsin and for S-100 protein. The proliferated histocytes of cosinophilic granutoma and Letterer-siwe disease were strongly immunoreactive for S-100 protein. In histiocytic medullary reticulosis (HMR) and in histiocytic lymphoma, all three markers were found within the tumor cells. ln fibrous histiocytoma and in juvenile xanthogranuloma, only a few weakly immunoreactive cells for S-100 protein were observed. lnflammatory malignant fibrous histiocytoma(MFH) (Xanthosarcoma) and xanthoma were immunoreactive for α 1-antichymotrypsin and lysozyme respectively. ln MFH of the storiform -pleomorphic type and in atypical fibroxanthoma, stains using all of the histiocytic markers were negative. These results suggest that eosinophilic granuloma. Letterer-Siew disease, fibroxanthoma and juvenile xanthogranloma are proliferative disorder of T-zone histiocytes; HMR and histiocytic lymphoma are those of pluripotential stem cells capable of dual histiocytic differentiation; xanthoma and xanthosarcoma are monocytic proliferative disease; and MFH of the storiform-pleomorphic type and atypical fibroxanthoma are not true histiocytic diseases.