The paramyxovirus simian virus 5 (SV5) cysteine-rich V protein has been shown to be a virus structural protein by analysis of the polypeptides of purified SV5 virions. In addition, the V protein has been identified as a component of the virus nucleocapsid core both by the analysis of the polypeptides present in radioactively labeled preparations of purified nucleocapsids and by immunoelectron microscopy. Quantitative autoradiography was used to determine that there are approximately 350 molecules of the V protein in virions. The V protein has been purified from V recombinant baculovirus-infected insect cells and by using inductively coupled argon plasma atomic emission spectroscopy it was found that each molecule of V binds two zinc atoms.