The oviduct-derived embryotrophic factor, ETF-3, enhances the development of trophectoderm and the hatching process of treated embryos. Monoclonal anti-ETF-3 antibody that abolishes the embryotrophic activity of ETF-3 recognized a 115-kDa protein from the conditioned medium of immortalized human oviductal cells. Mass spectrometry analysis showed that the protein was complement C3. Western blot analysis using an antibody against C3 confirmed the cross-reactivities between anti-C3 antibody with ETF-3 and anti-ETF-3 antibody with C3 and its derivatives, C3b and iC3b. Both derivatives, but not C3, were embryotrophic. iC3b was most efficient in enhancing the development of blastocysts with larger size and higher hatching rate, consistent with the previous reported embryotrophic activity of ETF-3. Embryos treated with iC3b contained iC3b immunoreactivity. The oviductal epithelium produced C3 as evidenced by the presence of C3 immunoreactivity and mRNA in the human oviduct and cultured oviductal cells. Cyclical changes in the expression of C3 immunoreactivity and mRNA were also found in the mouse oviduct with the highest expression at the estrus stage. Molecules involving in the conversion of C3b to iC3b and binding of iC3b were present in the human oviduct (factor I) and mouse preimplantation embryo (Crry and CR3), respectively. In conclusion, the present data showed that the oviduct produced C3/C3b, which was converted to iC3b to stimulate embryo development.Research is ongoing to optimize the embryo culture condition in human in vitro fertilization and embryo transfer programs. A large proportion of time when embryos are cultured in vitro is when they should be developing in the oviduct in vivo. Oviductal microenvironment is generally accepted to provide the best support to early embryo development. Oviductal cell coculture, the culture of embryo with oviductal cells, has been shown to improve the success rate in prospective randomized control clinical trials (1). In vitro experiments show that human oviductal cell coculture enhances the ability of the embryo at blastocyst stage to escape from its outer shell, zona pellucida, in a process known as hatching. Coculture also reduces the fragmentation rate of human embryos (2, 3) and increases the blastulation rate and total cell count per blastocyst of mouse embryos (4).Despite the success of coculture, it is not commonly used in in vitro fertilization and embryo transfer programs because of the complexity in its implementation as a service. Therefore, sequential culture, the use of different media for culturing embryos at different stages of development, is currently the method of choice for improving the outcome of the human in vitro fertilization and embryo transfer program (5). However, the culture condition in sequential culture systems is still suboptimal as the development of human embryos in these systems can be further improved by supplementation of granulocyte-macrophage colony-stimulating factor (6), a cytokine with peak expression during the preimplantation...