Background. Ras p21, a ras oncogene product, plays an important role in tumorigenesis, proliferation, and differentiation in various tissues and cells.

Methods. Using a monocolonal antibody raised against ras p21 (RASK 4), localization of ras p21 in normal epidermis and involved epidermis of various skin diseases was examined immunohistologically.

Results. Ras p21 was not present in basal cells of normal epidermis or basaloid cells of basal cell epithelioma but was found almost evenly in the cytoplasm of squamous cells and granular cells. This suggests that ras p21 is concerned with differentiation of epidermal cells. The mode of distribution of ras p21 differed from one cell to another in epidermal cells that turned to malignancy. The distribution was uneven and irregular in the tumorous region on the whole.

Conclusions. This result might possibly represent abnormal differentiation of epidermal cells that turned to malignancy, deviating from the regular mode of the distribution of ras p21, which is necessary for normal differentiation.