The level of expression of the cellular homologues to the oncogenes rasHa, rasKi, fos, myc, abl and raf was determined in chemically induced tumors, in mouse skin and in mouse epidermis after treatment with a tumor promoter. No significant increase in expression was found for any of the above proto-oncogenes. The expression of abl was reduced in both papillomas and carcinomas and after repeated applications of 12-O-tetradecanoylphorbol-13-acetate (TPA). Treatment of primary mouse epidermal cells with TPA in vitro failed to induce any marked alterations in proto-oncogene expression. Cell lines derived by treatment of epidermal cells with chemical carcinogens showed expression levels similar to untreated primary epidermal basal cells. These results suggest that increased expression of the proto-oncogenes under study is not required for tumor induction in mouse skin. The lower level of expression of abl in tumors and after repeated applications of TPA may indicate a role for this gene in some aspect of epidermal proliferation or differentiation.