We recently identified a new cytokine in the interferon (IFN) locus, which we designated IFNɛ because of its homology to other type I IFNs and capacity to bind to IFNAR and induce IFN regulated genes (IRGs). In contrast to other type I IFNs, which are only expressed after induction, usually by pathogens, IFNɛ is expressed constitutively in female reproductive organs and is not regulated by pathogen recognition receptor stimulation. Instead, it is regulated by ovarian hormones during the estrous cycle and pregnancy. In order to determine the function of this novel molecule, we generated mice with a null mutation in the Ifnε gene. Interestingly, numerous anti-bacterial IRGs expression in the uterus of Ifnε-/- mice were reduced, consistent with the idea that constitutive IFNε signaling maintains their expression at basal levels, which is presumably important for host defense. To determine the role of IFNɛ in genital tract infection, we examined the course and outcome of Chlamydia muridarum genital infection in Ifnε-/- mice. Intriguingly, Ifnε-/- mice had an earlier onset of disease and were unable to clear vaginal chlamydial infections at the same rate as WT mice. Compared to WT mice, Ifnε-/- mice exhibited severe clinical signs of infection, consistent with the increased bacterial load recovered by vaginal lavage throughout the course of infection. Importantly, reconstitution with rIfnε in C. muridarum-infected WT mice significantly reduced bacterial load. These data indicate that IFNɛ is important in maintaining constitutive innate immunity in the female reproductive tract and have potential implications for human urogenital infections.