The Ames test is a widely accepted short-term assay for detecting chemicals that induce mutations in the DNA of organisms. The assay is a reverse mutation assay that detects the mutational reversion of his-dependent Salmonella to his-independent colonies. Mutation-causing chemicals increase the frequency of occurrence of his-independent colonies, usually in a dose-dependent fashion. This unit includes protocols for the basic plate test with and without preincubation as well as procedures for highly volatile chemicals and gases, chemicals that require reductive metabolism, and chemicals that are available in small quantities. In addition there are protocols for dose selection, strain maintenance and preservation, freezing working and permanent cultures, and preparing the S-9 metabolic activation system.