PIP: The identification of human immunodeficiency virus (HIV)-2 has given rise to difficulties that were not anticipated when HIV-1 was thought to be the only virus involved in acquired immunodeficiency syndrome (AIDS). HIV-2, which seems to be prevalent only in Africa, differs from HIV-1 in its envelope and core proteins, and is more closely related to the simian AIDS virus than to HIV-1. Commercial assay kits that use HIV-2 antigen to detect anti-HIV-2 are available from only 1-2 manufacturers, although the cross-reactions between HIV-2 antibody and HIV-1 antigen probably allow most HIV-2 infections to be recognized by existing anti-HIV-1 assays. Until more HIV-2 infections are found in Europe and the US, unselected screening of blood donors for HIV-2 would be hard to justify. At present, testing of donors with West African connections and exclusion of those who have had sexual contacts in sub-Saharan Africa are probably sufficient measures to protect recipients in developed countries from HIV-2. More information is needed in 2 areas: HIV-2 infection in Africa and its effects, and the possible range of retrovirus infection in blood donors.