Human zygotes resulting from IVF were placed in two different culture systems to evaluate in-vitro development and to establish pregnancies in patients following embryo replacement. Treatment A (control) consisted of culturing zygotes in a modified Earle's Balanced Salt solution while treatment B consisted of culturing zygotes on a monolayer of fetal bovine uterine fibroblasts in this same culture medium. At the time of embryo replacement, embryos within treatments A and B had 3.7 and 4.3 blastomeres present, respectively. After 24 h in culture, the cellular fragmentation rate for treatment A embryos was 0.85 which was greater (P less than 0.05) than the fragmentation rate of 0.40 for embryos within treatment B. The incidence of implantation for patients whose embryos were given treatment A was 17.0% which was lower (P less than 0.05) than 35% for those given treatment B. Implantation rates increased with time in culture (43%) for treatment B embryos. Culture by treatment B of three-pronucleate zygotes resulted in 7/9 and 4/9 reaching the blastocyst and expanded blastocyst stages, respectively, whereas only 1/26 three-pronucleate zygotes cultured using treatment A reached either of these stages.