The cytoprotective effect of ulinastatin was studied in LLC-PK<sub>1</sub> cells treated with antimycin A, gentamicin, or cisplatin. All of the three agents induced a concentration-dependent increase in the release of lactate dehydrogenase and a decrease in the amount of remaining protein. In the cell injury models treated with 1.5 μ M antimycin A, 10 m M gentamicin, and 0.3 m M cisplatin, ulinastatin tended to show a cytoprotective effect at a concentration of 3,000 U/ml and provided a significant protective effect at 10,000 U/ml. LLC-PK<sub>1</sub> cells treated with 0.3 mM cisplatin, bovine serum albumin, and αi-acid glyco-protein at a concentration of 3.54 mg/ml, which is a comparable protein concentration to that of 10,000 U/ml ulinastatin, showed no protective effect but rather enhanced cell injury. These results suggest that ulinastatin exerts a direct protective effect on LLC-PK<sub>1</sub> cells against various renal toxicities.