A second retrovirus, named LAV-II, has been isolated from two West-African patients with AIDS. By its genomic sequences and its proteins, this virus is different from the LAV-I/HTLV-III, isolated from U.S.A., Europe and Central Africa. It differs also from STLV-III, isolated from Rhesus Macaques with AIDS, but displays an antigenic relationship with the latter virus, at the level of its external envelope protein. The tropism of LAV-II for T4 lymphocytes and the induction of cytopathic effect in infected cells are similar to those of LAV-I.