The presence of endogenous retroviral sequences (ERVs) in vertebrate genomes is well documented. ERVs have been found to be generally inactive and non-infective except in certain pathological conditions. However, baboon endogenous rectrovirus (BaEV) has been shown to be infective in vitro, and ERV particles have been isolated from normal placental villous tissues of most primates. These particles have ultra-structural features similar to type C retroviruses and are cross-reactive with antibodies to exogenous retroviruses such as HIV. Their function is not yet known. This review examines their possible role in modulation of the immune system during pregnancy and in the syncytiotrophoblast in primates.