Urinary trypsin inhibitor (UTI) is a serine proteinase inhibitor that is found in blood and urine. To investigate the physiological functions of UTI in vivo we generated UTI deficient mice by gene targeting. The mice showed no obvious abnormalities and appeared healthy. However, the females displayed a severe reduction in fertility. Wild-type embryos developed normally when transplanted into UTI-deficient female mice, suggesting that UTI-deficient females have a normal ability to maintain pregnancy. The number of naturally ovulated oocytes from UTI deficient mice was greatly reduced compared with that from wildtype mice. Histologically, oocytes with disorganized corona radiata were frequently seen in the ovaries of UTI-deficient mice after hormonal stimulation. When ovaries from UTI deficient mice were transplanted into wild-type mice, pups derived from the transplanted ovaries were obtained, suggesting that the ovary of UTI-deficient mice functions normally if UTI is supplied from the systemic circulation. These re suits demonstrate that UTI plays an important role in the formation of the stable cumulus oocyte complex that is essential for oocyte maturation and ovulation,