Keratins have been demonstrated to be suitable markers of changes taking place during epithelial neoplasia. Therefore, we analyzed 18 mouse skin tumors (nine papillomas and nine squamous cell carcinomas), induced either by two-stage carcinogenesis with 7,12-dimethylbenz[a]anthracene(DMBA)/12-O-tetradecanoylphorbol-13-acetat e or complete carcinogenesis with DMBA, by immunofluorescence with a monoclonal antibody to keratin (K) 8 (TROMA-1). Immunoperoxidase staining and immunoblotting were also used on selected tumor samples to further explore for the presence of K8. All of the papillomas tested were negative for the presence of K8, whereas the carcinomas were positive. The level of K8 expression in carcinomas showed a positive correlation with the degree of malignancy. Northern blot analysis using a K8 cDNA probe suggested that control of K8 expression in mouse skin tumors occurs at the transcriptional level. Double-label immunofluorescence staining using TROMA-1 and RK13 antibodies demonstrated that K8 did not generally colocalize with K13, a keratin normally found in internal stratified epithelial but aberrantly expressed in mouse epidermal tumors. Furthermore, tumors expressing high levels of K8 showed a reduced expression of K13. Histological examination of immunoperoxidase-stained tumors demonstrated that K8-positive cells were mainly found in anaplastic areas, whereas K13 foci were restricted to well-differentiated regions. Our results demonstrate that K8 expression is a marker of late stages of carcinoma progression in the mouse skin carcinogenesis model.