Human skin is known to contain protein-bound citrulline. This is the product of enzymatic deimination of arginine residues catalyzed by peptidylarginine deiminase. We probed frozen sections of human skin with a rabbit antiserum raised to rat skeletal muscle peptidylarginine deiminase using the avidin-biotin-peroxidase complex technique. This led us to interesting findings. No staining was observed in epidermis, inner root sheaths of hair follicles, sebaceous glands, and hair erector muscle. However, we noticed specific staining of the cytoplasm of secretory and myoepithelial cells of both eccrine and apocrine sweat glands. The procedure also stained neoplastic cells present in specimens dissected from extramammary Paget's disease. The data mean that peptidylarginine deiminase may be used as a new marker in the classification of skin neoplasms showing sweat gland differentiation. Possible localization of multiple types of peptidylarginine deiminases in human skin is discussed.