The aim of this study was to determine if the human pre-embryo produces a substance similar to the trophoblast interferon secreted by ruminant trophoblasts. Human embryos surplus to in-vitro fertilization (IVF) treatment were cultured up to 14 days following IVF. Viable cultures were determined by microscopic examination and by assay of the culture medium for human chorionic gonadotrophin (HCG). Four stages of development were visually identified: pre-blastocyst, unhatched, part-hatched and fully hatched blastocyst. HCG was detected in medium which had contained the more developmentally advanced embryos. A total of 62 samples were assayed for human interferon-alpha (IFN-alpha), including all cultures presumed viable. None contained detectable IFN-alpha immunoreactivity. Out of 14 candidate samples subjected to cytopathic effect reduction assay, none contained antiviral activity. We suggest that a trophoblast-derived interferon, unlike HCG, does not play a significant role in the maternal recognition of pregnancy in humans.