Since interaction with deoxyribonucleic acid (DNA) defines the genotoxic end points detected, simple in vitro tests can be used to monitor or screen most chemicals or mixtures of chemicals for specific genotoxic activity. Bacterial tests to screen classes of chemicals for gene mutations can provide a screen for many potential mutagens, carcinogens, and teratogens. Since the biochemical mechanisms for all mutagens, carcinogens, and teratogens are not the same, however, bacterial tests cannot be expected to detect all genotoxicants. Only by clearly understanding a test's usages and limits, can one most effectively use the information provided by that test.