The major proteins that are secreted by the preimplantation trophoblast of domestic ruminant species and that have been implicated as the an¬tiluteolytic factors modulating the release of the uterine luteolysin, pros¬taglandin F2a, during the period of maternal recognition of pregnancy have been identified as type I interferons (IFN). They are most appropriately named trophoblast IFN (IFNτ) and are related structurally to the IFNα, IFNβ, and IFNω with which they also share many biological properties.