To elucidate the role of reactive oxygen species (ROS) in the development of mouse embryo, effects of superoxide dismutase (SOD), catalase and erythrocytes were studied in vitro. Oocytes were fertilized and 2-cell-cleaved embryos were cultured in the presence or absence of either erythrocytes, SOD or catalase. Under standard culture conditions, the fertilization and cleavage rates were 77.4 and 12.5%, respectively. In the presence of xanthine and xanthine oxidase, those rates decreased to 28.2 and 4.5%, respectively. The hazardous effect of ROS was completely inhibited by erythrocytes. These results suggested that small amounts of erythrocytes might effectively degrade ROS during the development of cultured embryos.