CYCLOPHILINS are a family of proteins that bind the immunosuppressant cyclosporin A, possess peptidyl-prolyl cis-trans isomerase activity, and assist in the folding of proteins(1-6). Human cyclophilins A and B are host cell proteins that bind specifically to the HIV-1 Gag polyprotein p55(gag) in vitro(7). Here we report that viral particles formed by p55(gag), in contrast to particles formed by the Gag polyproteins of other retroviruses, contain significant amounts of cyclophilin A. Sequences in the capsid domain of p55(gag) are both required and sufficient for the virion-association of cyclophilin A. The association of cyclophilin A with HIV-1 virions was inhibited in a dose-dependent manner by cyclosporin A as well as by SDZ NIM811 ([Melle-4] cyclosporin), a non-immunosuppressive analogue of cyclosporin A(8). Drug-induced reductions in virion-associated cyclophilin A levels were accompanied by reductions in virion infectivity, indicating that the association is functionally relevant. Moreover, SDZ NIM811 inhibited the replication of HIV-1 but was inactive against SIVIMAC, a primate immunodeficiency virus closely related to HIV-1, which does not incorporate cyclophilin A.