The cellular localization of glial S-100 protein was investigated in paraffin-embedded sections of cutaneous histiocytosis X and in a variety of cutaneous infiltrative disorders, including juvenile xanthogranuloma, necrobiotic xanthogranuloma, papular xanthoma, eruptive histiocytoma, and reticulohistiocytosis. Immunoperoxidase staining with a rabbit anti-calf brain-S-100 antibody demonstrated strong and consistent activity in atypical histiocytes in all histiocytosis X specimens. No detectable S-100 protein was demonstrated in either histiocytes or giant cells in the non-X histiocytic disorders investigated. Positive controls were included within both groups of disorders in respect to melanocyte, epidermal Langerhans' cell, and dermal Schwann's cell staining. These findings are interpreted as evidence for diversity in the mononuclear phagocyte system and demonstrate the practicality of such a simple test in diagnostic problems involving infiltrative histiocytic disorders of the skin.