The invention relates to a new class of retroviruses, designated by HIV-2, of which samples have been deposited to the ECACC under numbers 87.01.1001 and 87.01.1002 and to the NCIB under numbers 12.398 and 12.399. It relates also to antigens capable to be obtained from this virus, particularly proteins p12, p16, p26 and gp140. These various antigens can be used for the diagnosis of the disease, especially by contacting these antigens with a serum of a patient submitted to the diagnosis. It relates to immunogenic compositions containing more particularly the glycoprotein gp140. Finally it concerns nucleotidic sequences, which can be used especially as hybridization probes, derived from the RNA of HIV-2.