The theanine (THE: γ-glutamylethylamide) content and the growth rate of cultured cells of tea ( Camellia sinensis L.) were increased greatly to 22.3%, in dry wt. with a medium containing 60 mM nitrate and 25 mM ethyl amine as a nitrogen source. The optimum concentrations of nitrate, Mg2 +, and K + for the growth and formation of THE in suspension cells were 40mM, 3mM, and 104mM, respectively. The yield of THE accumulated in the cultured cells with the medium modified for THE formation was increased greatly due to a great increase of the growth rate.