Intravenous and aerosol challenge experiments were conducted to evaluate tracheal and renal cross-protection in chickens immunized either with infectious bronchitis virus vaccine strains or with nephropathogenic infectious bronchitis virus (NIBV)-isolates. Intravenous challenge was carried out with four Belgian NIBV-isolates and with the Italian NIBV-strain AZ23.74. Virus titrations of tracheas collected at different times after aerosol challenge with the Belgian reference NIBV-strain B1648 demonstrated that only chickens immunized with the serologically closely related Belgian NIBV-field isolates showed strong trachea protection. The three vaccine strains H120, H52 and D274 and the heterologous NIBV-strain AZ23.74 did not induce protection of the respiratory tract against B1648. However, immunization with the latter strains shortened the period of replication of the challenge virus in the trachea. All chickens immunized with NIBV were strongly protected against nephritis after challenge with a homologous or heterologous NIBV-isolate. Vaccination with H52 gave partial kidney protection against NIBV challenge. Vaccination with H120 or D274 did not reduce kidney infection after NIBV challenge, as demonstrated by the incidence of mortality and kidney-immunofluorescence. Protection of the kidney was similar whether the challenge was performed intravenously or by aerosol. The results of both challenge experiments show that the development of a vaccine strain based on the Belgian NIBV-serotype is indicated to control NIBV in Belgian broiler flocks.