Tubulointerstitial nephritis was produced in 19 of 23 New Zealand White rabbits challenged i.v. with adjuvant-free homologous urine for greater than or equal to 16 wk and 11 of 14 challenged with adjuvant-free rabbit Tamm-Horsfall protein for 2 to 24 wk. Lesions were identical in the two groups of rabbits and were characterized by focal mononuclear infiltrates and microscopic scarring localized to distal nephron segments identified as the thick ascending limb of the loop of Henle. Concomitant immunoglobulin deposition was not detected despite antecedent elevations in serum IgG antibody directed against Tamm-Horsfall protein in 17 of 19 and 10 of 11 affected rabbits, respectively. Peripheral lymphocytes from affected rabbits were found to be cytotoxic and underwent blast transformation in the presence of homologous urine or Tamm-Horsfall protein in vitro. These lymphocytes were shown to produce a soluble cytotoxic product upon exposure to Tamm-Horsfall protein. Neither tubulointerstitial nephritis nor this pattern of cellular and humoral immune response to Tamm-Horsfall protein was found in two age-matched control groups: one unchallenged, and the other challenged i.v. with urine that had been selectively depleted of Tamm-Horsfall protein by 95%. It is concluded that the tubulointerstitial nephritis produced in rabbits by injection of urine or Tamm-Horsfall protein is the result of a predominately cellular immune response directed against Tamm-Horsfall protein.