An early response of the human and bovine endometrium to pregnancy is induction of an interferon (IFN)-stimulated gene (ISG) that encodes the ubiquitin-related protein, ISG15. Because the mode of implantation differs among species, we tested whether Isg15 mRNA was also expressed in the mouse uterus in response to the implanting conceptus. Isg15 mRNA was detected in the mouse uterus and increased after d 4.5 of pregnancy but did not change between d 3.5 and 9.5 of pseudopregnancy. Within the decidua, Isg15 mRNA was localized to the antimesometrial zone of the implantation sites. The level of Isg15 mRNA in artificially induced deciduomas was similar to the nonpregnant uterus and was approximately 10-fold lower than in the pregnant uterus. In vitro, murine decidual cells derived from artificially induced deciduomas could be induced to produce the Isg15 protein as well as Isg15-conjugated proteins when stimulated with type 1 IFN, though were less responsive to IFN-gamma. Isg15 is one of few gene products identified in murine implantation sites to require presence of the conceptus and not simply differentiation of the stroma. In vitro data support the inference that the pregnancy-specific inducer of uterine Isg15 is a type 1 IFN or a cytokine that signals through a similar pathway.