DNA polymerase beta is required in mammalian cells for the predominant pathway of base excision repair involving single nucleotide gap filling DNA synthesis. Here we examine the relationship between oxidative stress, cellular levels of DNA polymerase beta and base excision repair capacity in vitro , using mouse monocytes and either wild-type mouse fibroblasts or those deleted of the DNA polymerase beta gene. Treatment with an oxidative stress-inducing agent such as hydrogen peroxide, 3-morpholinosydnonimine, xanthine/xanthine oxidase or lipopolysaccharide was found to increase the level of DNA polymerase beta in both monocytes and fibroblasts. Base excision repair capacity in vitro , as measured in crude cell extracts, was also increased by lipopolysaccharide treatment in both cell types. In monocytes lipopolysaccharide-mediated up-regulation of the base excision repair system correlated with increased resistance to the monofunctional DNA alkylating agent methyl methanesulfonate. By making use of a quantitative PCR assay to detect lesions in genomic DNA we show that lipopolysaccharide treatment of fibroblast cells reduces the incidence of spontaneous DNA lesions. This effect may be due to the enhanced DNA polymerase beta-dependent base excision repair capacity of the cells, because a similar decrease in DNA lesions was not observed in cells deficient in base excision repair by virtue of DNA polymerase beta gene deletion. Similarly, fibroblasts treated with lipopolysaccharide were more resistant to methyl methanesulfonate than untreated cells. This effect was not observed in cells deleted of the DNA polymerase beta gene. These results suggest that the DNA polymerase beta-dependent base excision repair pathway can be up-regulated by oxidative stress-inducing agents in mouse cell lines.