A variety of co-culture systems have been devised to enhance the human embryo development in vitro. Vero cells were selected because they can be highly controlled and are easy to handle. To evaluate the embryotrophic effects of Vero cell monolayers, when they were co-cultured with human in vitro fertilized zygotes or subzonal inseminated oocytes. Total 1695 two-pronuclear embryos resulting from in vitro fertilization were cultured with Vero cells or medium alone for 24 hours. Similarly, sixty-six two-pronuclear embryos resulted from subzonal insertion of sperm (SUZI) with co-culture starting immediately following SUZI were compared with fifty-two two-pronuclear embryos resulted from SUZI, without co-culture. The numbers of blastomere and morphology of embryos were compared between the co-culture group and control group using student's t-test. Cell numbers in each embryo were greater in the IVF/co-culture group than in the control group (4.01 +/- 1.32 vs. 3.86 +/- 1.45, p < 0.05). The rates of poor quality embryo with major fragmentation were lower in the co-culture group than in the control group(11.5% vs. 19.9%, p < 0.001, for IVF embryos; 9% vs. 27%, p < 0.005, for SUZI oocytes). Co-culture SUZI oocytes on Vero cells prior to fertilization did not positively influence embryo cleavage, but improved embryo quality. We conclude that Vero cells can enhance human embryo development; however, the period for one-day or two-day co-culture is too short to provide a maximal support. Short term co-culture did not increase implantation rates. Immediate co-culture following SUZI might somewhat rescue the microinseminated oocytes. However, a longer duration of co-culture is necessary to exert the maximal effects on embryo development and implantation.