A contaminant was found to leach into the contents of two brands of disposable syringes. It was identified as 2-(2-hydroxyethylmercapto) benzothiazole and is believed to be formed during manufacture of the syringes as a result of a reaction between 2-mercaptobenzothiazole, a rubber vulcanization accelerator, and ethylene oxide, used for sterilization. The contaminant was isolated form the rubber plunger-seal and identified using mass, NMR, and UV spectroscopic methods. The amount of contaminant appearing in the contents of syringes was measured; up to 140 micro g was found under clinically relevant conditions. This finding has important implications with respect to the use of these syringes for drug administration and for the collection of blood for drug analyses.