A novel HIV type O immunodeficiency virus is disclosed which has the designation MVP-2901/94 and which has been deposited with the European collection of animal Cell Cultures (ECACC) under No. V 950121601. The characteristic antigens which can be obtained from the virus and which can be employed for detecting antibodies against retroviruses which are associated with immunodeficiency diseases are also disclosed, as are the partial DNA and amino acid sequences of the virus.