Interferon-tau (IFN-τ) is produced by the trophoblast prior to implantation in ruminants. It is involved in maternal recognition of pregnancy, and is a pleiotropic molecule that can alter the synthesis of endometrial proteins and inhibit proliferation of some cells. We have observed that IFN-τ reduces the DNA content in cultures of bovine endometrial epithelial cells; therefore, the objective of this study was to determine whether IFN-τ would induce apoptosis in bovine endometrial cells. Epithelial cells were prepared, cultured to confluence, and then incubated for 24 or 48 h in the presence or absence of 10 ng/ml progesterone, 100 ng/ml IFN-τ, or 10 μg/ml cycloheximide (CHX; an apoptosis inducer used as a positive control). Cells undergoing apoptosis exhibit such characteristics as the appearance of apoptotic bodies and DNA fragmentation. The incidence of apoptosis was assessed by using TUNEL, DNA fragmentation analysis, and Western blot analysis of Bax-α protein expression. The results showed that IFN-τ and CHX significantly increased the percentage of cells with apoptotic nuclei (33.6% and 44.8%, respectively) compared with controls (11.7%; P < 0.05). Progesterone treatment of the cells significantly inhibited the ability of IFN-τ to induce apoptosis (14.6%) compared with IFN-τ alone (33.6%; P < 0.05). DNA fragmentation analysis showed that INF-τ and CHX treatment resulted in an increase in the appearance of DNA laddering compared with that in untreated control cultures. Western blot analysis showed that IFN-τ and CHX treatment resulted in a greater expression of the proapoptotic protein Bax-α compared with that in control cultures. These data demonstrate that IFN-τ can induce apoptosis in bovine uterine epithelial cells and that this effect is modulated by progesterone. We speculate that IFN-τ might play a critical role in the remodeling of the endometrium around the time of implantation.