The ability of the sulphur compounds, N-acetyl cysteine, Methionine, and Glutathione to prevent inactivation of alpha 1-proteinase inhibitor by Myeloperoxidase-H2O2-Cl--system was investigated in vitro with purified components. The Myeloperoxidase system, or its main product HOCl by itself, readily abrogated the ability of alpha 1-proteinase inhibitor to inhibit elastase. This inactivation of alpha 1-proteinase inhibitor was effectively prevented by micromolar concentrations of N-acetyl cysteine, Methionine and reduced Glutathione, whereas oxidized Glutathione was much less effective. These results indicate that the sulphydryl compounds work as scavengers of the products of the Myeloperoxidase system, and might be useful in inflammatory disorders, to prevent tissue damage inflicted by this system.