Retrograde pyelonephritis was induced in inbred Fischer rat kidneys with Proteus mirabilis (1 × 10<sup>8</sup> organisms/ml). Sera from pyelonephritic animals sacrificed at 4 and 6 weeks contained cytotoxic antibodies to cultured syngeneic <sup>51</sup> Cr-labeled kidney tubular cells (p < 0.02 and p < 0.05, respectively) which could be absorbed with plasma membranes. Immune sera from 4- and 6-week pyelonephritic animals also displayed a granular fluorescent pattern along the surface of cultured kidney tubular cells. Quick-frozen syngeneic rat kidney sections stained positively with fluorescent antibody on the intraluminal side of kidney tubular cells. The results suggest that during chronic phases of pyelonephritis, auto-antibodies to kidney tubular cells are induced.