The seed oil of Jatropha curcas L. was shown to contain skin tumor promoters in a two-stage mouse carcinogenesis experiment. By using the irritant test on mouse ear to monitor activity, the "irritant fraction" was partially purified from the methanol extract of the seed oil by column chromatographies on Florisil and Sephadex LH-20. The irritant fraction obtained induced ornithine decarboxylase in mouse skin and inhibited the specific binding of 3H-12-O-tetradecanoylphorbol-13-acetate to a particulate fraction of mouse skin. After initiation with 7,12-dimethylbenz[a]anthracene (DMBA), this "irritant fraction" induced tumors in the skin of 36% of the mice tested in 30 weeks. Tumor incidences in the groups treated with DMBA alone and "irritant fraction" alone were 7% and 13% in week 30, respectively. Since the skin of Thai people comes into direct contact with this seed oil, an epidemiological study on human skin cancer in Thailand is indicated.