Techniques for culturing preimplantation mouse embryos from the two-cell stage to blastocyst are described, and the importance of this system to quality control assay the media and supplements used in human in vitro fertilization (IVF) procedures is discussed. Embryos from B6CBAF, mice were cultured in a commonly used mouse culture medium, modified Krebs-Ringer-bicarbonate (Krebs'), or in a commonly used human culture medium, Ham's FIO nutrient mixture supplemented with human fetal cord serum (FCS), and results were not significantly different. Using the mouse embryo culture system, tests on 174 preparations of FCS resulted in 24.1 % producing less than 75% morula or blastocyst stages after 72 h in culture, compared to 9.5% of the Krebs' control cultures. Rcsults of the mouse embryo culture system using 98 FCS subsequently used in human IVF were compared to results from the VIP Human In Vitro Program of Eastern Virginia Medical School of Norfolk, Virginia, from June 1982 through June 1983. The data suggest that prescreening of media and supplements using this mouse embryo culture system may indicate sources of factors potentially detrimental to the success of human IVF procedures.