Mice initiated with 7,12-dimethylbenz[a]anthracene (DMBA) and treated with 0.04% croton oil had more tumors and a greater percentage with tumors than a group of 0.004% croton resin-treated mice. In a second experiment addition of corn oil to the croton resin increased the number of tumors. Skin treated with DMBA and croton oil had greater peroxidation and more tumors than skin treated with DMBA and croton resin. Peroxides could not be detected throughout the experiment. Peroxidation in 2 experiments increased after DMBA initiation but returned to normal on about day 45. Antioxidants applied on days 2–21 in another experiment, when peroxidation increased, decreased the tumor incidence. At the conclusion of the DMBA-croton oil versus the DMBA-croton resin experiments, small tumors regressed more than large tumors. Regression of tumors observed at 16–18 weeks may be linked to an increase of lysosomal enzyme activity observed before regression.