The keratin polypeptide composition of different types of warts was studied by means of SDS polyacrylamide gel electrophoresis. It was composed of five main polypeptides, similar to those found in normal epidermis (mol. wt. 67K to 55K) and an additional polypeptide (72K) was often detected. The distribution of each polypeptide was expressed as percentage of total keratins in the different samples. The most characteristic feature observed in warts was the marked decrease of the 67K polypeptide. There were increased amounts of the 67K-63K proteins and some difference was observed in the distribution of these proteins in the different types of warts. Thus, the viral infection seems mainly to modify the proteins which are only involved in the latest stages of the differentiation of keratinocytes.