This assay may be used to measure gene mutation in yeast, a eukaryotic micro-organism. Strains of Saccharomyces cerevisiae have been developed which detect forward or reverse mutations. A variety of haploid and diploid strains of the yeast can be used to measure the production of gene mutations induced by chemical agents (solid, liquid, vapour or gas). Stationary or growing cells are exposed to the test chemical with and without an exogenous mammalian metabolic activation system for up to 18 hours at 28°-37°C with shaking. After incubation for 4-7 days at 28°-30°C in the dark, plates are scored for survival and the induction of gene mutation. If the first experiment is negative, then a second experiment should be carried out using stationary phase cells. If the first experiment is positive, it is confirmed in an appropriate independent experiment. At least 5 adequately spaced concentrations of the test substance should be used. At least 3 replicate plates should be used per concentration for the assay of prototrophs produced by gene mutation and for viability.