The trophoblasts interferons (IFN) are Type I IFN with about 50% amino acid sequence identity to the leukocyte IFN (IFN-α). They are the major secretory products of the trophoblast of ruminant ungulate species during pregnancy in the period immediately preceding attachment and implantation when they have been implicated in the phenomenon known as maternal recognition of pregnancy. The trophoblast IFN have antiviral and antiproliferative activities typical of other Type I IFN, but unlike IFN-α, -β and -ω are poorly responsive to viral induction and have a highly restricted pattern of expression. Nevertheless, a recombinant bovine IFN-α can mimic many of the propeties of the trophoblast IFN and has been used pharmacologically to improve pregnancy success in sheep. It still remains unclear, however, whether the trophoblast IFN have unique biological properties or whether they are unusual merely by virtue of the location, magnitude and temporal nature of their expression at a critical time during pregnancy.