Antisera suitable for detection of SIVSM or SIVMAC Vpr proteins on Western blots of purified virions are currently not available. We have expressed the Vpr protein of SIVSMPBj1.9 in a gst-based prokaryotic expression system and used it to raise polyclonal antisera in rabbits. Two immune sera were obtained that specifically recognized both cell- and virion-associated Vpr protein on immunoblots of three different SIV isolates (SIVSMPBj1.9, SIVMACBK28, and SIVMAC239). Because Vpr is believed to play an important role in HIV/SIV replication and pathogenesis, these reagents will allow the extension of functional analyses of this protein to a broader spectrum of viruses. Both antisera and the gst-Vpr expression plasmid have been submitted to the NIAID AIDS Research and Reagent Program and are available to interested investigators.