The premalignant evolution of chemically induced mouse skin papillomas is characterized by dysplastic changes, aneuploidy, induction of γ-glutamyl transpeptidase (GGT), and changes in the expression of keratins, especially differentiation-associated K1. This keratin, which is expressed in normal epidermis and early papillomas, is no longer present in more advanced dysplastic and aneuploid papillomas and in fully invasive carcinomas. More recently, it has been shown that K13, a keratin normally present in internal epithelia but not in epidermis, is aberrantly expressed in epidermal tumors. In the present study, the timing of expression of K13 and its correlation with other markers of premalignant evolution were investigated. Papillomas were induced by SENCAR mice by a single initiating dose of 20 nmol of 7,12-dimethylbenz[a]-anthracene (DMBA) and promotion with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) (2 μg twice a week). Tumors were randomly harvested at 10, 20 and 35 weeks of promotion. K13 and K1 expression in papillomas was studied using immunoblotting and immunostaining of consecutive sections, as previously described. As expected from previous studies, the distribution of Kl in papillomas collected at 10 weeks of promotion was restricted to differentiated cells and was uniform throughout the section of the papilloma. Conversely, K13 was expressed only as small foci in 10 out of 21 papillomas (48%). Papillomas of 20 weeks were also positive for Kl. Staining for K13 was positive in these papillomas with the exception of only one that was essentially negative, presenting only one small positive focus. Some of the papillomas collected at week 35 were negative for Kl, but immunostaining with K13 showed uniform staining of suprabasal cells in all the papillomas studied. In all cases, immunohistochemical results were confirmed by immunoblotting with proteins extracted from 7 μm sections from each paraffin block. These results indicate that keratins K1 and K13 are coexpressed in most papillomas from 10 to 35 weeks of promotion. However, analysis of adjacent sections showed that K13 positive areas are topographically located in the Kl negative areas of the papillomas, suggesting a shift in the differentiation program from epidermal to mucosal types of keratinization. Based on these and previous studies from our laboratory, we conclude that K13 is an early marker of papillomas progression, which occurs before gross chromosomal abnormalities are present in the stem line of the tumors, and precedes dysplastic changes and the onset of GGT expression, and is probably concomitant at the individual cell level with loss of K1.