We have developed a genetic system, termed IVET (in vivo expression technology), designed to identify bacterial genes that are induced when a pathogen infects its host. A subset of these induced genes should include those that encode virulence factors, products specifically required for the infection process. The system is based on complementation of an attenuating auxotrophic mutation by gene fusion, and it is designed to be of use in a wide variety of pathogenic organisms. In Salmonella typhimurium, we have successfully used the system to identify a number of genes that are induced in BALB/c mice, and that, when mutated, confer a virulence defect. The IVET system has several applications in the area of vaccine and antimicrobial drug development. The technique was designed for the identification of virulence factors and thus may lead to the discovery of new antigens useful as vaccine components. The IVET system facilitates the isolation of mutations in genes involved in virulence and, therefore, should aid in the construction of live attenuated vaccines. In addition, the identification of promoters that are optimally expressed in animal tissues provides a means of establishing in vivo regulated expression of heterologous antigens in live vaccines, an area that has been previously problematic. Finally, we expect that our methodology will be used to uncover many biosynthetic, catabolic, and regulatory genes that are required for growth of microbes in animal tissues. The elucidation of these gene products should provide new targets for antimicrobial drug development.