Interferon-tau (IFN tau) is the pregnancy recognition signal of bovine embryos, inhibiting luteolysis. We studied trophoblastic growth and IFN tau secretion of embryos with different developmental potential, i.e., in vivo derived and in vitro produced embryos, cloned embryos and demi-embryos, to evaluate if the ability of secreting IFN tau might be responsible for differences in pregnancy rates after transfer of these categories of embryos to recipients. Day 8 embryos of excellent quality were individually placed in microdrops of buffalo rat liver cell-conditioned medium and maintained for up to 23 days. Embryos were observed on Days 11, 15, 19 and 23, the mean diameter (2r) of attached and spherical embryos was measured, and their trophoblastic area was calculated as r(2)pi or 4r(2)pi, respectively. Simultaneously, medium was changed and the IFN tau levels of conditioned media were determined using a bioassay of antiviral activity. Trophoblastic area was smaller (P < 0.05) in demi-embryos than in all other groups, which exhibited similar trophoblastic growth until Day 19. However, on Day 23 trophoblastic area of in vivo derived embryos was more than twice (P < 0.05) as large as those of in vitro produced and nuclear transfer (NT) embryos. IFN tau levels increased only slowly with time in culture of demi-embryos. By contrast, the level of IFN tau doubled from Day 11 to Day 15 in conditioned media from all other groups of embryos. The linear increase in IFN tau production of vivo and in vitro derived embryos continued until the end of the culture period, whereas conditioned media from NT embryos contained significantly (P < 0.05) less IFN tau activity on Days 19 and 23 than those of the former two groups. Our results demonstrate different capabilities of secreting IFN tau for in vivo derived and in vitro produced embryos vs. NT and demi-embryos, which may-at least part-be responsible for the differences in pregnancy rates after transfer to recipients. (C) 1999 Elsevier Science B.V. All rights reserved.