The baboon oviductal epithelium differentiates into a tall columnar epithelium consisting of ciliated and secretory cells during the follicular phase of the menstrual cycle in response to rising oestradiol levels. The apical tips of these secretory cells are filled with membrane-bound secretory granules. During the luteal phase when progesterone levels are elevated, the epithelium regresses and deciliation occurs. Analysis of secretory proteins obtained from explant culture media by SDS-PAGE followed by fluorography or Western blots has revealed that the baboon oviduct synthesizes and secretes a high molecular weight glycoprotein during the follicular phase of the cycle. Immunocytochemistry demonstrated that this oviductal glycoprotein is localized to the secretory granules of epithelial secretory cells, is oviduct specific, and that following secretion the oviductal glycoprotein binds to the zona pellucida and perivitelline space of ovulated oocytes and embryos within the oviduct. Similar proteins have been characterized in other mammalian species. cDNA data show that the complete coding sequence is 2228 bp for a protein of 623 amino acids. A Genbank search showed that baboon oviductal glycoprotein has high homology to other oviductal glycoprotein sequences at both the nucleotide and amino acid levels. Studies conducted to date probing the biological function of oviductal glycoprotein indicate that this protein plays a role in prefertilization reproductive events (sperm capacitation; sperm-zona binding; zona penetration). Additional experiments are needed to reveal a specific function and mechanism for this molecule.