In this study, the influences of cumulus cells during in vitro fertilization (IVF) and their subsequent effects on mouse embryo development in vitro were studied in inbred and hybrid strain mice. Embryos at the 2-cell stage obtained after in vitro fertilization were transferred to the oviduct of foster mothers to assess their implantation rate. Mouse oocytes, which were intact at the time of in vitro fertilization, demonstrated less polyspermic fertilization and more blastocyst development than those inseminated without any cumulus cells. The data demonstrate that more oocytes were arrested at the 2-cell stage in groups that contained no cumulus cells. There was no statistical difference between cumulus-enclosed and cumulus-free groups for embryo implantation. Data suggest that although there was no difference in terms of implantation rate, more blastocyst stage embryos were obtained from cumulus cell enclosed oocytes. Therefore, we suggest that cumulus cells play many roles during in vitro fertilization, and they should be intact during in vitro fertilization.