Cod Gadus morhua and bovine trypsin degraded human epidermal keratin with similar efficacies in vitro around optimal pH, which was at pH 8.4 for cod trypsin and at pH 9.5 for bovine trypsin. Extract of intestines of cod, Atlantic herring Clupea harengus, Atlantic salmon Salmo salar, and redfish Sebastes marinus degraded keratin with similar efficacies with pH optima between 8.5 and 9.5. Sheets of plantar callus were degraded with somewhat lower efficacy than keratin. The keratin-degrading activity of extract of cod intestines had a temperature optimum around 45 degrees C. Inhibition with benzamidine and 4-phenylbutylamine showed that trypsin amounted to more than 2/3 of the keratin-degrading activity in all extracts of fish intestines. Apart from cod intestines, which had the lowest chymotrypsin content, chymotrypsin made a smaller but significant contribution to the keratin-degrading activity. The present investigation demonstrates that fish trypsin and extract of fish intestines are effective in degrading human epidermal keratin in vitro, and in a recent investigation the same was shown with fish pepsin. This may suggest a possible mechanism for the development of irritative contact eczema caused by exposure to fish.