The method follows the release of alanine from .gamma.glutamyl alanine by a series of linked reactions that result in the oxidation of NADH to NAD. The pH optimum for human erythrocyte .gamma.GCT [glutamyl cyclotransferase] was 9.0, and the Km for .gamma.glutamyl alanine was 2.3 .times. 10-3M. Human erythrocytes had the highest activity of all the species studied (human, rabbit, dog, sheep, cattle, chicken).