Replication of human immunodeficiency virus type 1 (HIV-1) in non- dividing cells depends critically on import of the vital preintegration complex into the nucleus. Recent evidence suggests that viral protein R (Vpr) plays a key regulatory role in this process by binding to karyopherin α, a cellular receptor for nuclear localization signals, and increasing its affinity for the nuclear localization signals. An in vitro binding assay was used to investigate the role of Vpr in docking of the HIV-1 preintegration complex (PIC) to the nuclear pore complex. Mutant HIV-1 PICs that lack Vpr were impaired in the ability to dock to isolated nuclei and recombinant nucleoporins. Although Vpr by itself associated with nucleoporins, the docking of Vpr+ PICs was dependent on karyopherin β and was blocked by antibodies to β. Vpr stabilized docking by preventing nucleoporin-stimulated dissociation of the import complex. These results suggest a biochemical mechanism for Vpr function in transport of the HIV-1 genome across the nuclear pore complex.