We discover that epigallocatechin gallate (EGCG) from green tea is an inhibitor of fatty-acid synthase (FAS) from chicken liver. Its inhibition of FAS is composed of reversible fast-binding inhibition, through which 52 muM EGCG can inhibit 50% of the activity of FAS, and irreversible slow-binding inactivation following saturation kinetics with the dissociation constant of 0.352 mM and limiting rate constant of 0.0168 min(-1). The marked inhibition of ketoacyl reduction shows that the inhibition is related to beta -ketoacyl reductase of FAS. The observable protection of NADPH and competitive inhibition of NADPH for ketoacyl reduction indicate that EGCG may compete with NADPH for the same binding site. The synthetic inhibitor C75 does not show obvious fast-binding inhibition, but does exhibit irreversible slow-binding biphasic inactivation, which is demonstrated to be a second-order reaction. That the inactivation by C75 is protected by malonyl-CoA indicates C75 is similar to cerulenin in being a covalent inactivator of the beta -ketoacyl synthase.