Objective : The purpose of the current series of experiments were to assess the effect of GM-CSF, as a medium supplement, on the development of mouse embryos and the expression of LIF and IL-1? mRNA. Materials and Methods: Mouse 2-cell embryos were collected from the oviducts of 6 weeks old ICR mice at 48 hours after hCG injection. Embryos were cultured in P-1 medium supplemented with mouse GM-CSF (0, 1, 5, 10 ng/ml). The embryo development to blastocysts and hatching blastocysts was assessed and the cell number in blastocyst was also examined. Using RT-PCR, the expressions of LIF and IL-1? mRNA in blastocyst were evaluated in the GM-CSF supplemented group and control group. Results: In mouse, the addition of GM-CSF increased the percentage of blastocysts (65.5%, 68.6%, 73.0% and 76.1% for control and 1, 5 and 10 ng/ml, respectively), and increased the proportion of hatching blastocysts (35.2%, 36.4%, 43.2% and 53.0% for control and 1, 5 and 10 ng/ml, respectively). The mean cell numbers in blastocyst were significantly increased in GM-CSF supplemented groups compared to control group. LIF and IL-1? expression in blastocyst were significantly higher in GM-CSF supplemented group than in control group. Conclusion: The results of experiment by mouse embryos showed beneficial effects of GM-CSF as a medium supplement. Furthermore, the addition of GM-CSF significantly increased the expression of LIF and IL-1? in mouse embryos. These results suggest that GM-CSF might be a important molecule in embryo implantation.