Transient and massive production of ovine trophoblast protein-1 (oTP-1) by preimplantation conceptuses seems to be a critical event required for the establishment of successful pregnancy. We have previously demonstrated that one of several oTP-1 genes is predominantly expressed between days 13 and 20 of pregnancy and that this oTP-1 gene contains an AP-1 site, a transcription enhancer element, in the 5'-flanking region. We have obtained evidence, indicating a linkage between cytokine granulocyte macrophage-colony stimulating factor (GM-CSF) and conceptus production of the trophoblast interferon (IFN), oTP-1. These are: 1) oTP-1 production (both polypeptide and mRNA) is enhanced by the addition of GM-CSF in vitro and 2) GM-CSF mRNA is localized in the luminal and glandular epithelium of the uterine endometrium. Based on these observations, we propose that the massive amounts of oTP-1 produced during the period of pregnancy establishment is stimulated at least in part by maternal GM-CSF.