In spite of widespread use of antibacterial agents to control colibacillosis in poultry, the development of an effective vaccine has been an important goal for researchers. Compared to bacterins and subunit vaccines, live vaccines are attractive because they are much cheaper than purified proteins, and can be used for mass immunization via aerosol, feed, or drinking water. Our goals in this research were to construct non-revertible mutant derivatives of avian pathogenic strains of E. coli and to evaluate their potential ability to protect chickens against experimental disease. cya crp mutants...