The proliferating cell in histiocytosis X has been thought to be an abnormal Langerhans cell since the identification in 1965 of the characteristic X-bodies morphologically identical to the Birbeck granules of epidermal Langerhans cells. This conclusion is based primarily on this finding and on morphologic similarities at the light microscopic level. Enzyme histochemical data have been somewhat conflicting and have not provided strong support for this conclusion. Recently, immunohistochemical studies have shown data consistent with a Langerhans cell origin for histiocytosis X cells. This study documents the presence of nonspecific esterase, acid phosphatase, and adenosine triphosphatase in both Langerhans cells and histiocytosis X cells. Both types of cells also react with antibodies directed against widely distributed leukocyte antigens (HLA-A,B,C; Ia; L3B12); Langerhans cell/thymocyte antigen [Leu 6(T6)]; histiocyte antigen (Leu-M3); helper T-cell/histiocyte antigen [Leu-3(T4)]; and S-100 protein. These results complement earlier immunologic studies and add enzyme histochemical data that strongly support the concept of histiocytosis X as a proliferative disorder of cells of Langerhans lineage.