Theanine (gamma-glutamylethylamide) is the major "umami" (good taste) component of tea and its favorable physiological effects on mammals have been reported. An enzymatic method for the synthesis of theanine involving bacterial gamma-glutamyltranspeptidase (GGT) was developed. The optimum reaction conditions were 200 mM Gln, 1.5 M ethylamine, and 0.4 units/ml GGT, pH 10. After 2-h incubation at 37 degreesC, 120 MM theanine was obtained, the conversion rate against Gln being 60%. Theanine was purified on Dowex 50 W x 8 and Dowex I x 8 columns, and then identified by H-1-NMR. This is the first report that a GGT, regardless of its source, can utilize an alkyl amine as an acceptor of the gamma-glutamyl moiety in its transpeptidation reaction.