Asynchrony between embryo and uterine environment is one of the major limits in human in-vitro fertilization (IVF). A culture system which could prolong culture time and increase embryonic cleavage rate and viability would improve success rates. Using Vero cells, an in-vitro co-culture system was developed to investigate and promote human embryo development. Vero cells provide good support for human early embryos up to the blastocyst stage. When fertilized embryos were co-cultured, 68% of them reached the blastocyst stage. Pregnancy rate was 50% per transfer in patients with several previous failures of implantation. A significant increase in clinical pregnancy rate was also demonstrated when zygotes were maintained on Vero cell monolayer for only 24 h. The beneficial effect of the feeder layer may be through the release of embryotrophic factors and the detoxification of the culture medium by the cells. Co-culture is a new concept in assisted reproduction.