Corynebacterium renale type I (strain 115), 1.7 X 10(7) to 4.5 X 10(7) organisms, introduced intravenously into mice disappeared from the blood less than 24 h after inoculation and did not produce pyelonephritis. The same strain, 1 X 10(7) to 5 X 10(7) organisms, inoculated into the urinary bladder of mice was not recovered from the blood in any of the mice, but caused pyelonephritis accompanied by ureteritis and cystitis in 16 of 21 (76%) mice. Pyelonephritis and cystitis in mice were histopathologically similar to those found in cows. The antibody response was observed only in the mice with pyelonephritis or pyelitis, but not in those with only cystitis or in those without lesions, as found in cows. Similar diseases were produced in mice by C. renale types II and III but less frequently than by type I. It is suggested, therefore, that mice may be useful in the study of bovine C. renale infection.