Interferon tau (IFNτ) is the pregnancy recognition signal produced by the conceptus trophectoderm and acts in a paracine manner on the ovine endometrium to increase expression of IFN-stimulated genes primarily in the stroma and deep glandular epithelium, including IFN regulatory factor-1 (IRF-1). The roles of Stat1, Stat2, and IRF-9 in IFNτ regulation of IRF-1 expression were determined using human stromal fibroblasts lacking specific IFN signaling components or complemented with specific Stat1 mutants. In parental (2fTGH) cells treated with IFNτ, Stat1α/β was tyrosine phosphorylated by 15 min, and IRF-1 mRNA and protein increased from 0 to 6 h, was maximal at 6 h, and decreased to 24 h. In contrast, IFNτ did not affect IRF-1 expression in Stat1- and Stat2-deficient cells or in Stat1-deficient cells complemented with Stat1 Y701Q or Stat1 R602L mutants. In Stat1-deficient cells complemented with the Stat1 S727A mutant, Stat1α, or Stat1β and treated with IFNτ, IRF-1 increased from 0 to 6 h, was maximal at 6 h, and decreased thereafter. In IRF-9-deficient cells stimulated with IFNτ, IRF-1 increased from 0 to 6 h but did not exhibit the sharp decline from 6 to 12 h observed in other cells. Collectively, results indicate that IFNτ effect on IRF-1 expression is primarily regulated by tyrosine-phosphorylated Stat1α or Stat1β dimers, whereas the decline of IRF-1 after 6 h of IFNτ treatment is regulated by IRF-9.