A one-electron reductive metabolism of 1,2-dibromoethane (DBE) is described that gives rise to a free radical intermediate, which can be stabilized by a spin trapping agent and detected by electron spin resonance spectroscopy. Using rat liver microsomes or isolated hepatocytes from phenobarbitone pretreated animals, under hypoxic conditions, it has been possible to trap a free radical intermediate and identify it by using 13C-DBE. Inhibition experiments have demonstrated that the site of activation is the microsomal drug metabolizing system.