IFNs are produced by conceptus and/or placental tissues in several mammalian species. Of these IFNs, the trophoblast interferons, oTP-1 and bTP-1, are clearly the most well characterized and have been found to be members of an unusual 172-amino-acid-long IFN-alpha subfamily. Although classified as IFN-alpha IIs, they are unique in two respects. First, the 3' non-coding regions of their mRNAs differ from those of other IFN-alpha s and, secondly, oTP-1 and bTP-1 are expressed in extraordinarily large amounts during a defined period of early pregnancy. oTP-1 and bTP-1 have physiological actions which are clearly anti-luteolytic, although it is suspected that they are not the only conceptus products required to maintain the corpus luteum of pregnancy. A role for trophoblast interferons in local regulation of the uterine immune system is also anticipated. Because IFNs are known to exhibit other activities, including effects on cell proliferation, cell differentiation, and the induction of specific gene transcription in target cells, the trophoblast and/or placental interferons may also be influencing uterine function by such mechanisms. However, this question remains largely unexplored. Finally, the regulation of expression of the trophoblast interferons represents an important area of research that has the potential to lead to significant reductions in the incidence of embryonic loss in mammals.