A member of the human endogenous retrovirus (HERV) family termed HERV-W encodes a highly fusogenic membrane glycoprotein that appears to be expressed specifically in the placenta. It is unclear whether the glycoproteins of the HERVs can serve as functional retrovirus envelope proteins to confer infectivity on retrovirus particles. We found that the HERV-W envelope glycoprotein can form pseudotypes with human immunodeficiency virus type 1 virions and confers tropism for CD4-negative cells. Thus, the HERV-W env gene represents the first HERV env gene demonstrated to encode the functional properties of a retrovirus envelope glycoprotein.