(−)-Epigallocatechin gallate (EGCG) is the main polyphenolic constituent of extracts of green tea. EGCG slightly inhibited specific binding of [3H] 12-0-tetradecanoylphorbol-13-acetate ([3H]TPA) to a particulate fraction of mouse skin, and caused a prompt decrease in the phorbol ester receptor number in mouse skin. It also inhibited the activation of protein kinase C by teleocidin. In week 25 of a two-stage carcinogenesis experiment in mouse skin, tumors were found in 53% of the mice treated with 7,12-dimethylbenz(a)anthracene (DMBA) plus teleocidin, but in only 13% of those treated with DMBA plus teleocidin and EGCG. The average number of tumors per mouse in these groups was 2.1 and 0.1, respectively. No tumors were found in other groups treated with DMBA alone or teleocidin and EGCG alone. Thus EGCG inhibited tumor promotion by teleocidin.