We have recently described a novel type 1 interferon (IFN) co-expressed with IFN-γ by the trophectoderm of the pig conceptus between day 12 and day 18 of gestation, a development stage that corresponds to implantation in the uterus. This IFN, now officially named IFN-δ, is recognized as the first member of a novel type 1 IFN family. This paper reviews the main published data on IFN-δ, together with some new data, showing that IFN-δ, while being a true type 1 IFN, has some very specific structural and biological properties. Sequences related to IFN-δ coding sequence were found in the genome of man and other ungulates but the only other potentially functional gene was found, so far, in the horse. The pig IFN-δ mature protein, with 149 amino acids, is the smallest of all known type 1 IFNs. It is unusually rich in cysteines (seven residues), and has a very basic isoelectric point. Recombinant IFN-δ expressed in insect cells is glycosylated and has a high antiviral activity on porcine cells, but not on human cells. It has high antiproliferative activity, which is significantly enhanced in the presence of IFN-γ. This new IFN was shown to bind on pig cells to the same type 1 receptor as IFN-α. IFN-δ and IFN-γ genes are co-regulated in the pig trophectoderm, whose cells on day 14–16 of development simultaneously secrete both IFN proteins. The biological role of porcine IFN-δ in early pregnancy has been found unrelated to the known antiluteolytic effect of trophoblastic IFN-τ in ruminants.