Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) is a 15kDa regulatory protein packaged in the HIV-1 virion. Although the molecular mechanism of Vpr function during viral replication remains elusive, Vpr has been found to possess interesting biological activities, including cell-cycle arrest at the G2/M check point, promotion of the HIV-1 pre-integration complex for nuclear transport, and a low but significant level of transcriptional activation of a variety of viral and cellular promoters. We now present data suggesting that HIV-1 Vpr is a nucleic-acid-binding protein. This activity of Vpr was demonstrated by DNA-cellulose chromatography, antibody co-immunoprecipitation, and gel electrophoretic mobility shift assays. By mutational analysis, the C-terminal region of Vpr, which is rich in basic amino-acid residues, was shown to be critical for Vpr binding to nucleic acids. The nucleic-acid-binding activity of Vpr is consistent with several biological activities of Vpr and may provide an important clue for understanding the molecular interactions between HIV-1 and the host cells.