The goals of this project are to understand HIV-1 double infection and its impact on recombination and viral evolution. Cells infected by two or more genetically different viruses can produce heterozygotic virions, from which recombinant viruses are generated. Therefore, double infection directly affects the overall recombination rate. We have studied the nature of HIV-1 double infection in cultured and primary T cells by direct cell-free virus infection and by cell-mediated virus transmission. We have observed that HIV-1 double infection occurs significantly more frequently than it would at random in both transmission pathways. This is the first study that reveals the nature of retroviral double infection. We are now dissecting the mechanisms that cause nonrandom HIV-1 double infection.