MutY specifies a DNA glycosylase that removes adenines unnaturally paired with various bases including oxidized derivatives of guanine, such as 7,8-dihydro-8-oxoguanine (8-oxoG). The rate of mutation in starved Escherichia coli cells is markedly raised in mutY mutants defective in this glycosylase. As predicted, the mutations produced include G to T transversions. Bacteria carrying mutM or fpg-1 mutations (defective in Fapy glycosylase, which removes oxidized guanine residues such as 8-oxoG) show little or no enhancement of mutation under starvation conditions. When present together with mutY, however, mutM clearly further enhances the rate of mutation in starved cells. Plasmids resulting in overproduction of MutY or Fapy glycosylases reduce the rate of mutation in starved cells. We conclude that, in non-growing bacteria, oxidized guanine residues, including 8-oxoG, constitute an important component of spontaneous mutation. Addition of catalase to the plates did not reduce the mutant yield, indicating that extracellular hydrogen peroxide is not involved in the production of the premutational damage. Singlet oxygen, known to give rise to 8-oxoG, may be the ultimate oxidative species.