During early pregnancy in ruminants, the embryo not only prevents prostaglandin F_2α release, but it also modifies protein synthesis in the endometrium. This is accomplished by the secretion of interferon-tau (IFN-τ) from the embryo. The objective of this study was to identify and characterize specific proteins secreted from endometrial epithelial cells in response to IFN-τ that could be important for endometrial function and/or embryo development. The epithelial cells were prepared and cultured to confluence and then incubated with or without 100 ng/ml IFN-τ. At the end of the incubation, the proteins in the medium were analyzed by two-dimensional PAGE. The result showed that two major protein spots were induced by IFN-τ. One has a molecular mass of approximately 12 kDa and an isoelectric point (pI) of 6.7; the other has a molecular mass of 76 kDa and pI of 4.8. Protein sequence analysis showed that the 12-kDa protein contained a partial amino acid sequence that corresponded to macrophage migration inhibitory factor (MIF). To determine whether MIF is expressed in endometrial cells, isolated stromal or epithelial cells were incubated with or without 100 ng/ml IFN-τ for 0, 3, 6, 12, 24, and 48 h. After incubation, the MIF protein in cells was examined by Western blotting analysis, and the steady-state mRNA for MIF was examined by Northern analysis. Results showed that MIF protein and mRNA were present in the epithelial cells but not the stromal cells. The presence of MIF in the luminal epithelium of endometrial tissue was confirmed by immunohistochemistry. However, there was no effect of IFN-τ on MIF expression in the epithelial cells. The concentration of MIF in the medium was quantified by Western blotting analysis to determine if IFN-τ altered MIF protein secretion from the epithelial cells. The results showed that IFN-τ significantly stimulated the secretion of MIF protein from the cells. These data show that MIF is expressed in the epithelial, but not the stromal, cells of the endometrium and that MIF secretion from the epithelial cells is stimulated by IFN-τ. It is therefore likely that MIF plays a role in early embryo development, and further characterization of MIF expression and its regulation in the endometrium will add significantly to our understanding of early embryo-uterine interactions.