Neuropsin is a trypsin-type serine protease that was first cloned from the mouse brain as a factor related to neural plasticity. Subsequent in situ hybridization histochemical analysis indicated a broad localization of its mRNA throughout the whole body, although the details remain obscure. In this study, we showed that neuropsin immunoreactivity is localized in the keratinized stratified epithelia of the mouse epidermis, hair, tongue, palate, nasal cavity, pharynges, esophagus, and forestomach. In the skin and mucous membranes, neuropsin immunoreactivity was found in the stratum spinosum and the stratum granulosum. The immunoreactivity in the former sublayer was mainly present in the cytoplasm, but that in the latter sublayer was exclusively present in the intercellular space or on the outer surface of the cell membrane and thus exhibited a lamellar-like peripheral distribution. During development, the appearance of neuropsin immunoreactivity in the various epithelia was found at embryonic days 14.5-15.5, prior to formation of the stratum corneum. More extensive expression of neuropsin immunoreactivity was found in the nude mouse skin and mucous membranes than in wild-type mice. Because the nude mouse is characterized by genetic impairment of keratinization, such abnormal neuropsin expression might be caused or affected by this impairment. Therefore, neuropsin, an extracellular serine protease, is suggested to be involved in keratinization in the stratified epithelia.