Murine renal tubular epithelial cells and interstitial fibroblasts may express both Fas (CD95) death receptor and Fas ligand and are vulnerable to Fas-mediated death in vitro. We therefore hypothesized that an absence of renal Fas may protect resident cells from undergoing apoptosis. We performed unilateral ureteric ligation [producing unilateral ureteral obstruction (UUO)] in 6-wk-old normal control mice and C57Bl6/lpr mice, which express a nonfunctional Fas receptor. Obstructed kidneys were removed atdays 3,7, and14 (n= 6 per group). Tubular cell apoptosis at day 7 was significantly reduced inlpr mice [21.8 ± 5.8 vs. 45.7 ± 7.6 cells/10 high-power fields (hpf),P < 0.02]. Importantly, there was no difference in tubular cell proliferation between normal andlpr mice at any time point studied. Interestingly, double labeling with terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) and the proximal tubule-specific antibody Fx1A indicated that the absence of Fas reduced distal but not proximal tubular death atday 7. In addition, there was no difference in interstitial cell apoptosis or proliferation, suggesting that Fas does not play a significant role in interstitial cell death. Importantly, inflammatory macrophage infiltration and ultimate collagen I deposition was unchanged in lprmice. In conclusion, the absence of functional cell surface Fas in UUO provides distal tubular cells with partial protection from apoptosis but does not affect interstitial cell fate in this model of tubulointerstitial injury.