A stable isotope dilution liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneously quantifying glutathione (GSH), glutathione disulphide (GSSG) and glutathione sulphonamide (GSA) from biological samples. GSA is a selective product of the reaction of GSH with hypochlorous acid and a potential biomarker of myeloperoxidase activity. GSH was detected as the N-ethylmaleimide alkylated adduct, as formation of this species prevented GSH oxidation occurring during sample processing. Synthesised stable isotope analogues were used as internal standards to accurately quantify each target species. The limit of quantification was determined as being 0.1pmol for each species and excellent linearity was observed over relevant concentration ranges for biological samples. Relative standard deviations were <5% for within-day variation and <10% for between-day variation, except at the lower limit of quantification where they remained <20%. Accuracy was between 82% and 113%. We could detect GSA in neutrophils and endothelial cells treated with hypochlorous acid and in bronchoalveolar lavage fluid from children with cystic fibrosis. This is the first time GSA has been quantified in clinical material and suggests it is formed in vivo. The assay can now be used for investigating GSA as a biomarker of myeloperoxidase activity in inflammatory conditions, and is also applicable to measuring GSH:GSSG molar ratios as a general index of oxidative stress.