The effect of mixed live vaccines of Newcastle disease (ND) and infectious bronchitis (IB) on specific pathogen-free chickens aged 7 days was investigated. The chickens were inoculated intranasally with mixed live vaccines with and without Escherichia coli, or with E. coli alone. "Vaccine 1" consisted of ND virus strain B1 and IB virus strain ON; "vaccine 2" consisted of ND virus strain B1 and IB virus strain H120. The tracheas of chickens inoculated with the vaccines and E. coli and with the vaccines alone showed multiplication of E. coli and histological lesions (loss of cilia, degeneration and hyperplasia in epithelial cells and cellular infiltration of subepithelial tissues). In the tracheas of chickens inoculated with vaccines and E. coli, multiplication of E. coli was greater than in chickens given vaccine alone. There were no histological lesions, and only mild, transient multiplication of E. coli in chickens inoculated with E. coli alone. The results suggest that these mixed live vaccines, especially vaccine 2, play a role in inducing or enhancing colibacillosis in the chicken.