Glutathione is an intracellular reducing agent. It is synthesized by a two step reaction catalyzed by gamma-glutamylcysteine synthetase (GCS) and glutathione synthetase. GCS, which is the rate limiting enzyme in the synthesis of glutathione, is inhibited specifically by buthionine sulfoximine (BSO). The enzyme was partially purified from the parasitic nematode Brugia malayi, the causative agent of lymphatic filariasis. BSO inhibits the enzyme activity in an irreversible manner. The Brugia enzyme was found to be 24-fold more sensitive in depleting the glutathione contents of the parasites with low concentration. The results may conclude that GCS is a potential target for the development of drugs against filariasis.