The ability of the infectious bronchitis (IB) Ma5 and 4/91 live-attenuated vaccines to protect against kidney damage caused by a nephropathogenic strain of IB virus (B1648) was investigated. Protection parameters considered were gross and microscopic renal pathology, and the use of a polymerase chain reaction to detect IB RNA in kidney tissue. By each parameter, Ma5 vaccine alone provided poor protection, but 4/91 alone or the combined program both protected well.