Retrograde pyelonephritis was produced in rats by the intravesical injection of Proteus mirabilis. When animals were preimmunized against Proteus mirabilis by (a) prior infection, (b) administration of antigen, or (c) passively transferred antiserum, they were resistant to infection by proteus when challenged by the retrograde route. The protective effect of specific preimmunization in retrograde pyelonephritis indicates that a major site of action is retardation of bacterial growth within the parenchyma of the kidney.