Four strains of infectious bronchitis virus (IBV) were compared in chickens as to nephropathogenicity. Two of the viruses were from the United States, one was from Australia, and one from Italy. Each has been described as nephropathogenic. The Australian strain proved to be most pathogenic and was then followed by the Italian strain and, finally, the two viruses from the United States. With an increase in the age of the chickens there was an apparent decrease in pathogenic effect. All of the viruses were highly pathogenic in the respiratory tract regardless of the age of the test birds. Kidney damage, or nephritis, was detected more readily on histological study rather than by gross examination. Each of the viruses was unrelated to the others when the sera were assayed by virus neutralization. Massachusetts and Connecticut IBV vaccines gave erratic, or only, partial protection against challenge form the respective viruses which confirmed their aberrant nature. Lesions of IBV infection in the respiratory tract did not necessarily correlate with the results from challenge virus recovery attempts. One Massachusetts-type IB vaccine strain, Holland IBV, proved capable of inciting nephritis under the conditions of this study and from inoculating specific-pathogen-free chickens at 2 days of age. The lower embryo passage of this strain proved more nephropathogenic than the higher ones. Other Massachusetts-type vaccine viruses and the Connecticut-type did not induce nephritis. The nephropathogenic potential for certain vaccine strains is discussed.