We have developed a quantitative forward mutation assay using Salmonella typhimurium, in which resistance to the purine analog 8-azaguanine is used as a genetic marker. We present the assay protocol, the concentration-dependent toxicity and mutagenicity of five known mutagens (N-methyl-N'-nitro-N-nitrosoguanidine, ICR-191, 9-aminoacridine, dimethylnitrosamine, and benzo[a]pyrene), and reconstruction experiments testing the assay for possible bias. The relative merits of forward versus reverse mutation assays are discussed.