Studies from this laboratory have shown great diversity in the glycosylation of tissues comprising the interhaemal barrier of species with different placental types. This diversity may be one of the factors preventing interbreeding between species. Glycan expression within the uterine epithelium and trophoblast of the interhaemal barrier was examined to test this proposition in three species with similar diffuse, microcotyledonary, epitheliochorial allantochorionic types of placenta: the horse (Equus caballus) and donkey (Equus asinus), which can interbreed with each other, and the camel (Camelus dromedarius), which cannot interbreed with either of the other two species. A panel of 14 lectins was used and it was found that glycosylation patterns were generally similar between placental tissues of the horse and donkey, except for the expression of non-bisected complex N-glycan and some sialic acids, whereas those of the camel showed striking differences in the binding of lectins to many structures carrying terminal residues of fucose, N-acetyl galactosamine and beta-galactose, as well as to complex N-glycans and sialic acids. These results are consistent with the proposition that interbreeding species carry similar glycans in tissues forming the interhaemal barrier whereas glycodiversity is one of the factors preventing implantation and subsequent placental development in interspecific hybrids.