This study was designed to develop the in vitro culture systemof mammalian preimplantation embryos. We proposed mouse fetal fibroblast cells (MFFC) from 14∼15 day mouse fetus. Zygotes from superovulated female ICR mice were cultured 96 hrs in simple defined media (T6) or on the monolayer of MFFC. In addition, to evaluate the action of the co-culture of MFFC, various diluted superoxide dismutase antibody (SOD-Ab) was supplemented into the monolayer of MFFC and zygotes were cultrued in presence or absence of SOD-Ab. The developmental rates of zygotes were significantly increased in co-culture with MFFC compared to the control. The rates of zygotes to the 4-cell stage in media treated with EDTA were higher than those cultured in MFFC but the proportions of morula and blastocyst were not differ between EDTA and MFFC. Interestingly blastocysts in co-culture with MFFC possessed as many as blastomere as those developing in vivo, but blastocysts cultured with EDTA had significantly fewer blastomeres. In addition, the treatment of SOD-Ab suppressed the beneficial effect of MFFC. Therefore, our findings suggest that co-cultrue system using MFFC may have an advantage in the development of mouse zygotes as well as embryonic differentiation.