Previous work had shown that HIV-1 is a large group of highly divergent viruses, which mutate quickly in the infected person. The analysis of two HIV-2 strains by molecular cloning and sequence analysis has now proved that HIV-2 is a large and very divergent family of viruses as well and that genetically different subtypes exist in the same individual. Both, HIV-1 and HIV-2 strains form subtypes with remarkable differences in their growth potential on lymphocytes and macrophages. Such properties may change during the persistent infection. An antiviral chemotherapy thus must be active against HIV-1 and HIV-2, it must inhibit the viruses on lymphocytes as well as on macrophages and should begin early, in order to avoid that new HIV-subtypes arise in the patient.