The objective of this Phase I/II study was to assess the potential for green tea to be used as a colorectal cancer chemopreventive agent. This study measured the dose-related biological effects of administration of a single dose of green tea on the rectal mucosa of normal volunteers. Volunteers were admitted to the Robert Wood Johnson Medical School Clinical Research Center for 24 h. Baseline blood and rectal biopsy samples were obtained before the volunteers drank 0.6, 1.2, or 1.8 g of green tea solids dissolved in warm water. Blood samples were taken 2, 4, 8, and 24 h after the tea administration. Rectal biopsies were obtained at 4, 8, and 24 h. Prostaglandin E2 (PGE2) levels were analyzed by ELISA. Tea polyphenol levels in the blood, urine, and rectal tissue were measured by high-performance liquid chromatography using a Coulochem electrode array detection system. Statistical comparisons were made using ANOVA. Decreased levels of PGE2 in rectal mucosa were observed at 4 and 8 h after consumption of green tea. There was no correlation between inhibition of PGE2 and tissue or plasma levels of tea polyphenols. Ten of 14 subjects demonstrated a response to green tea, as evidenced by at least a 50% inhibition of PGE2 levels at 4 h. We conclude that green tea constituents have biological activity in inhibiting PGE2 synthesis. Given the 71% "response rate," we believe these data support the study of green tea as a colorectal chemopreventive agent in more long-term Phase II trials.