Successful pregnancy requires suppression of maternal immune response to the implanting conceptus, which acts as a semiallograft. During the implantation period in humans and rodents, various immune modulators are produced at the maternal-fetal interface and regulate functions of cytotoxic T cells and NK cells for protection of conceptuses from the maternal immune system. However, maternal immune responses to the conceptuses during the establishment and maintenance of pregnancy are not much understood in pigs which show true epitheliochorial type placentation. Previously, we reported that SLA-DQ molecule, a type of MHC class II molecules, is expressed in the uterine endometrium during pregnancy in a stage- and cell type specific manner, and that SLA-DQ expression is essential for the maintenance of pregnancy. Thus, to understand the role of SLA-DQ and maternal-fetal immune interaction, we examined expression of CD80 and CD86, co-stimulators for T cell activation, in the uterine endometrium during pregnancy. We also measured levels of CD80 and CD86 mRNAs in the uterine endometrium of pigs carrying conceptuses derived from somatic cell nuclear transfer (SCNT) and those from natural mating on Day 12 of pregnancy. Expression of endometrial CD80 mRNA was affected by day of pregnancy, and levels of CD80 mRNA were significantly higher on Day 15 of pregnancy than those of the estrous cycle. Expression of CD86 mRNA did not change during pregnancy. Levels of CD80 and CD86 mRNAs were not different in the uterine endometrium of pigs carrying SCNT derived conceptuses on D12 of pregnancy compared to those with conceptuses derived from natural mating. These findings suggest that CD80 and CD86 are involved in immune interactions at the maternal-fetal interface during pregnancy for the establishment and maintenance of pregnancy in pigs.