The main physiologically active polyphenol in green tea extract is (-)-epigallocatechin gallate (EGCG). Green tea extract has an advantage over EGCG as a cancer chemopreventive agent for humans, as is apparent from the Japanese custom of injesting green tea on a daily basis. Green tea extract similarly inhibited protein kinase C activation by teleocidin, a tumor promoter, as did EGCG. In addition, EGCG and green tea extract showed inhibitory effects on the growth of lung and mammary cancer cell lines with similar potencies. An experiment using the estrogen-dependent MCF-7 cell line showed the mechanisms of action of these compounds to be inhibiting the interaction of estrogen with its receptors. Considering our previous results of a single application of EGCG to mouse skin inhibiting the specific binding of 3H-12-0-tetradecanoylphorbol-13-acetate (3H-TPA) and 3H-okadaic acid, we postulated that EGCG and compounds in green tea extracts would block the interaction of tumor promoters, hormones and growth factors with their receptors: a kind of sealing effect. The sealing effect would account for reversible growth arrest, and may be induced by various kinds of compound.