The effects of human or mouse leukemia inhibitory factor(hLIF or mLIF) were examined as a means of increasing the development of in vitro matured(IVM) and in vitro fertilized (IVF) oocytes into morulae or blastocysts. Cell numbers of blastocysts were also counted using Hochest dye staining. Two-to 8-cell embryos derived from bovine IVM/IVF oocytes were cultured 5 to 6 days in CRI aa with or without mLIF or hLIF. All culture media were contained 3mg/ml bovine serum albumin. In experiment 1, the proportion of embryos developed to morulae and blastocysts in CRI aa containing 5,000U/ml mLIF(37.8%) was slightly higher than those of CRIaa containing 1,000U/ml mLIF(34.6%) and 0 U/ml mLIF(27.4%; P>0.05). In experiment 2, 0, 1,000 and 5,000U/ml of hLIF added to CR1aa media yielded 27.6%, 43.0% and 35.5% morulae and blastocysts, respectively(p>0.05). These were no significant increases in cell number among treatments(p>0.05). These results were indicating that mLIF or hLF can increase the proportion of embryos that develop into morulae and blastocysts without and increase in the cell number.