The effect of endogenous glutathione (GSH) on the induction of DNA double strand breaks (dsb) by 25 MeV electrons was investigated using stationary haploid yeast cells defective in γglutamyl-cysteine-synthetase (gsh 1) containing less than 5 per cent of the normal GSH content. In gsh 1 cells the induction of dsb is increased by a factor of 1·5 under oxic and 1·8 under anoxic irradiation conditions: whereas the oxygen enhancement ratio was only slightly decreased (1·9) compared to wild-type cells (2·4).