Although the role of exfoliative toxin in staphylococcal scalded-skin syndrome has been suggested to be that of a serine protease, it has not been demonstrated to show proteolytic activity. Our purpose was to purify a proteolytic enzyme from a mixture of exfoliative toxin and newborn-mouse epidermis. We used gel filtration and ion-exchange and hydroxyapatite chromatography with a high-pressure liquid chromatography system. A casein-hydrolyzing enzyme was isolated from the mixture. The molecular mass of the enzyme was confirmed to be 20 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Subcutaneous injection of the purified enzyme into newborn mice reproduced the epidermal splitting that is seen in staphylococcal scalded-skin syndrome. These results suggest that exfoliative toxin does not work as a protease itself but that some reaction between exfoliative toxin and an epidermal component(s) first produces a protease, after which epidermal splitting occurs.