Retroviruses replicate by inserting their proviral DNA into host cell chromosomes, usually in cells of the immune system. Nevertheless, an efficacious and safe subunit recombinant vaccine has been prepared against the feline leukemia retrovirus of cats. Because of high rates of morbidity and mortality, particularly in the developing world, the development of a vaccine against human immunodeficiency virus type I (HIV-I) has become very important. The very high rate of mutation or variation of lentiviruses, such as HIV-I, makes a vaccine against this virus particularly difficult. Active research must be encouraged to design a modified envelope vaccine molecule of gp120 subtype E or C for trials in appropriate developing country settings.