We studied the therapeutic effects of human urinary trypsin inhibitor (UTI) in 5 cases with acute renal failure, resulting from traumatic shock in 1 case, post-operative shock in 2 cases, septic shock in 1 case, and dehydration in 1 case. We administered 300,000 u/day of UTI intravenously at the initial phase of acute renal failure for 7 days. We measured the activities of urinary N-acetyl-beta-D-glucosaminidase (NAG), alanine aminopeptidase (AAP) and the activities of serum beta-glucuronidase, PMN-elastase serially. As a control, we also studied same markers in 5 cases with acute renal failure without the administration of UTI. We could obtain the following results. 1) Urinary activities of NAG and AAP were already elevated markedly at the onset phase of acute renal failure. 2) The administration of UTI caused a significant decrease of the activities of NAG and AAP in the urine as compared with those in the controls. 3) The administration of UTI caused also the significant suppression of the activities of beta-glucuronidase and PMN-elastase in the serum. These results suggested that UTI has the protective effects on the tubular epithelial cell injuries in cases of acute renal failure.