The corpus luteum (CL) is essential for establishing pregnancy. If pregnancy does not occur during the estrous cycle, luteolysis is induced by prostaglandin (PG) F2alpha secreted from the uterus. Galectin-1, abeta-galactose-binding protein, is expressed in the functional CL of cows and increases the viability of bovine luteal steroidogenic cells (LSCs) by modifying the functions of membrane glycoproteins. The binding of galectin-1 to glycoproteins is blocked by alpha2,6-sialylation of the terminal galactose residues of glycoconjugates, which is catalyzed by a sialyltransferase (ST6Gal-I). However, the physiological role of alpha2,6-sialic acid in bovine CL is unclear. The level of alpha2,6-sialylation of the bovine CL was higher during the regressed-luteal stage than in other luteal stages. Lectin histochemistry revealed that alpha2,6-sialylated glycoconjugates were localized to luteal endothelial cells throughout the estrous cycle. In addition, alpha2,6-sialylated glycoconjugates concentrated to the membrane of LSCs during the regressed-luteal stage. PGF2alpha treatment for 72 h enhanced the expression ofST6Gal-ImRNA and the level of alpha2,6-sialylated glycoproteins in mid-LSCs. The level of alpha2,6-sialylated glycoproteins of late-stage LSCs (Days 15-17 after ovulation) was higher than that of mid-stage LSCs (Days 8-12 after ovulation), and galectin-1 increased the viability of mid-LSCs but not that of late-stage LSCs. Furthermore, galectin-1 increased the viability of late-LSCs when alpha2,6-sialic acid residues were removed by neuraminidase. The overall findings suggest that alpha2,6-sialylation stimulated by PGF2alpha contributes to luteolysis by inhibiting the luteotropic effects of galectin-1 in bovine CL.