This study compared the infections of chickens with virulent and avirulent strains of avian infectious bronchitis virus (IBV). Sixty 3-week-old chickens were infected by aerosol with a virulent strain of IBV and 60 with an avirulent strain. Immunofluorescent staining of impression smears and cryostat sections of the trachea revealed that the virulent IBV infected and destroyed the ciliated epithelial cells lining the trachea. The virus then became localized in subepithelial cells and was detectable for 21 days. The avirulent IBV infected only a few ciliated epithelial cells and there was no significant desquamation of the epithelium. The avirulent IBV, like the virulent, became localized in subepithelial cells and was detectable for 24 days. The immunologie response to virulent and avirulent IBV, as measured by virus neutralization titers, was similar at 19 days post infection. After this time, there was a divergence of the responses, with the virulent virus inducing ascending antibody titers while the antibody titers descended with the avirulent virus.