In a field trial the efficacy of a New Zealand Infectious Bronchitis (IB) vaccine in the forty-seventh egg passage was evaluated. The vaccine, administeredviathe drinking water, had only a very mild residual pathogenicity for IB susceptible andMycoplasma gallisepticum-free birds. Immunity was challenged three weeks after vaccination by intratracheal inoculation with four different New Zealand types of IB virus. All groups of vaccinated chickens showed much less severe clinical and histological signs of respiratory disease than control chickens, and challenge virus was re-isolated from a lower number of the vaccinated birds. These results indicate that the vaccine effectively immunized the birds against all four challenge viruses.