1. Samples of urine from two human subjects accidentally exposed to sulphur mustard were analysed for metabolites derived from hydrolysis (thiodiglycol, thiodiglycol sulphoxide), conjugation with glutathione (1,1'-sulphonylbis [2-S-(N-acetylcysteinyl)ethane]) and from further metabolism of glutathione conjugates by the beta-lyase pathway (1,1-sulphonylbis[2-(methylsulphinyl)ethane], 1-methylsulphinyl-2-[2-(methylthio)ethylsulphonyl]ethane). 2. Thiodiglycol sulphoxide was excreted in much higher concentrations than thiodiglycol, as was observed previously in rat exposed to sulphur mustard. However, the use of thiodiglycol sulphoxide as a biological marker for sulphur mustard poisoning is limited by its presence at low concentrations in normal human urine. 3. beta-lyase metabolites were detected at concentrations comparable with those of thiodiglycol sulphoxide. No background levels of beta-lyase metabolites have been detected in normal human urine, and they are proposed as unequivocal diagnostic and forensic indicators of sulphur mustard poisoning in man.