Vertebrate DNA contains numerous genomes closely related to retroviruses, i.e. endogenous retroviruses. While most of the retroviruses are pathogenous, endogenous retroviruses have rarely been shown as such. Endogenous retroviruses, as most of the retroviruses, are able to cross 'species barriers'. Porcine endogenous retroviruses were described in 1974. They are expressed in tissues and cells involved in transplantation, endothelial cells, for instance. They are capable of interspecific transmission, expressed in vitro in human cells; no evidence of in vivo interspecific transmission has been reported so far. As far as xenotransplantation is concerned, porcine endogenous retroviruses represent a risk of adaptation to humans of a new form of retrovirus. Such a risk mandates a close monitoring of recipients and their partners. This risk has already been taken with the use of tissues and stable blood-derived products from animals harboring in their genomes endogenous retroviruses.