A nephrotoxicity study of Cefmatilen hydrochloride hydrate (S-1090) was conducted in rabbits at single oral doses of 250, 500 and 1000 mg potency/kg. All treated groups showed a decreased food consumption and a tendency for the body weight to decrease. Urinary protein and glucose were detected and slight increases of plasma creatinine and urea nitrogen were observed in the 500 mg potency/kg group. Urinary protein was also detected in the 1000 mg potency/kg group. In the histopathological examination of the kidney, tubular necrosis was observed in the 500 and 1000 mg potency/kg groups. No nephrotoxic signs were observed in the 250 mg potency/kg group. The NOAEL on the nephrotoxicity of S-1090 in rabbits was estimated to be 250 mg potency/kg.