Acetic acid, a very weak tumor promoter in the multistage mouse skin model for chemical carcinogenesis, was found to be very effective at enhancing cancer development, when applied during the progression phase of the model. Papilloma-bearing mice when repeatedly treated with acetic acid had a greater carcinoma incidence and a greater conversion of papillomas to carcinomas than vehicle treated mice. Selective cytotoxicity is discussed as a possible mechanism.