A study has been done of immunization against Streptococcus faecalis pyelonephritis in the rat. Intravenous, subcutaneous, and bladder immunizations were tested with intravenous and retrograde challenge. Logarithmic and stationary growth phase organisms were used separately for both immunization and challenge. The only instance of significant, but incomplete, protection was in animals immunized subcutaneously or intravenously and challenged intravenously with log phase organisms. No correlation was found between circulating antibody response and infection. Serum bactericidal activity was not increased by immunization or infection.