Toll-like receptor 4 (TLR4) is present on monocytes and other cell types, and mediates inflammatory events such as the release of TNF after exposure to LPS. C3H/HeJ mice are resistant to LPS-induced mortality, due to a naturally occurring mutation in TLR4. We therefore hypothesized that LPS-induced acute renal failure (ARF) requires systemic TNF release triggered by LPS acting on extrarenal TLR4. We injected C3H/HeJ mice and C3H/HeOuJ controls with 0.25 mg of LPS, and sacrificed them 6 h later for analysis of blood urea nitrogen (BUN) and kidney tissue (n= 8 per group). In contrast to C3H/HeOuJ controls, C3H/HeJ mice were completely resistant to LPS-induced ARF (6-h BUN of 32.3±1.1 vs 61.7±5.6 mg/dl). C3H/HeJ mice released no TNF into the circulation at 2 h (0.00 vs 1.24±0.16 ng/ml), had less renal neutrophil infiltration (6.4±1.0 vs 11.4±1.3 neutrophils per high power field), and less renal apoptosis, as assessed by DNA laddering. Transplant studies showed that C3H/HeJ recipients of wild-type kidneys (n= 9) were protected from LPS-induced ARF, while wild-type recipients of C3H/HeJ kidneys (n= 11) developed severe LPS-induced ARF (24-h BUN 44.0±4.1 vs 112.1±20.0 mg/dl). These experiments support our hypothesis that LPS acts on extrarenal TLR4, thereby leading to systemic TNF release and subsequent ARF. Renal neutrophil infiltration and renal cell apoptosis are potential mechanisms by which endotoxemia leads to functional ARF.