The properties of four serotypes of New Zealand Infectious Bronchitis (IB) virus in the 10th and the 40th to 50th egg passage were investigated. All viruses were chloro-form-sensitive. Three types in the 10th egg passage were inactivated by heating to 56°C within 10 minutes, the fourth one within 30 minutes. The growth curve of all viruses at a low and a high passage level has been determined.