In the first part, the biosynthesis of mercapturic acids is described and attention is payed to the source of the cysteine moiety in those acids, namely glutathione. The second part deals with the present state of knowledge concerning the conjugation of xenobiotics with glutathione, catalyzed by the several several enzymes of the glutathione S-transferase system. The role of the glutathione S-transfereases in the metabolism of xenobiotics is the subject of the third part, emphasizing their functions in detoxification : enzymatical catalysis with glutathione, reversible binding to hydrophobic compounds, and covalent binding to reactive electrophiles. Finally, a recent hypothesis is described with respect to the activation of vicinal dihalogen compounds to reactive agents.