Mutagens and carcinogens in the environment represent a potential hazard, and it is important to screen large numbers of compounds and mixtures of compounds to which humans are exposed. However, it is Impractical for both technical and monetary reasons to do the bulk of mutagen screening by using mammals. Carcinogenicity testing in mammals is also extremely expensive and takes years for adequate tests. The Salmonella test we have developed, and the other short-term tests that have been developed for testing chemicals for their ability to interact with DNA, or for mutagenicity, are being widely used and should help in solving some of the problems that cannot be adequately approached by human epidemiology or animal cancer tests alone.