In ruminants, interferon produced by the trophectoderm (IFN-τ) is recognized as the embryonic signal responsible for maternal recognition of pregnancy. IFN-τ is believed to act by down-regulating estrogen receptors, thus preventing appearance of oxytocin receptors responsible for the release of prostaglandin F_2α (PGF_2α) by the endometrium. The present study was undertaken to determine in vitro the biological activities of different IFN-τ isoforms and document putative alternate luteotrophic mechanisms. Endometrial cells in primary cultures were treated with five different rIFN-τ isoforms: two ovine isoforms (ro-4 and ro-11) and three bovine isoforms (rb-1a, rb-2b and rb-3b). Their effect was quantified by measurement of PGE₂ and PGF_2α production by ELISA and induction of cyclooxygenase (COX-2) by Western and Northern analysis and correlated with antiviral activity previously reported. The overall pattern of response to the IFNs tested suggests that low concentrations (<1 μg/ml) reduced the production of both PGs and higher concentrations (>1 μg/ml) stimulated preferentially PGE₂; however, exceptions were noted. Isoform rb-2b with high antiviral activity inhibited PG production in both cell types at all concentrations tested. IFNs rb-1a and ro-11 had similar antiviral activities, inhibiting PG at low concentrations and stimulating them at high concentrations. Isoform rb-3b stands out relative to the other IFNs tested because it induced a variable non-dose-dependent effect on PG production and low antiviral activity. An increase in COX-2 protein expression and messenger was correlated with increased PG production. The results showing two distinct responses to IFN-τ depending on its concentration and/or isoform and the absence of correlation with antiviral activity suggest that complex transduction mechanisms are involved.