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Publisher Summary This chapter presents the assay procedure and properties of δ-(α-Aminoadipyl) cysteinylvaline Synthetase. δ-(α-Aminoadipyl) cysteinylvaline synthetase differs from glutathione synthetase, which is found mainly in the supernatant fraction obtained by centrifugation after ultrasonic treatment of the mycelium. The enzyme is assayed by measurement of the incorporation of 14 C from labeled valine into δ- (α-aminoadipyl) cysteinylvaline in the presence of δ- (L-α-aminoadipyl)-L-cysteine. The synthesis of δ-(L-α-aminoadipyl)- L-cysteine has been described by Loder and Abraham. The labeled product is isolated by paper electrophoresis and chromatography, after oxidation to the sulfonic acid form, and identified by comparison with an authentic sample of the tripeptide. δ- (L-α-aminoadipyl)-L-cysteinyl-D-valine has been synthesized by Usher. The disulfide of the corresponding all-L tripeptide has been synthesized by Rudinger.