Our laboratory has identified a new facet of human immunodeficiency virus type 1 (HIV-1) entry. We demonstrated that the incorporation of host cyclophilin A (CypA) into nascent viruses is absolutely required for HIV-1 attachment to target cells. Although CypA is initially incorporated into the interior of the virus, we found that during maturation CypA relocates to the viral surface. Our work indicates that exposed CypA mediates HIV-1 attachment to target cells via heparans. We believe that this interaction between CypA and heparan represents the initial step in HIV-1 entry.