Velvet tobacco mottle virus (VTMoV) and Solanum nodiforum mottle virus (SNMV) are serologically closely related as shown by immunodiffusion tests. Dissociated VTMoV coat protein separates into three electrophoretic components in polyacrylamide gels corresponding to polypeptides with Mr's of about 37,000, 33,000 and 31,500, whereas SNMV protein migrates as a single component of Mr about 31,000. Evidence is presented indicating that the VTMoV particle is built from a single species of protein subunit of Mr about 37,000. However, the protein is unstable and degrades during virus purification into a stable polypeptide of Mr about 31,500. It is proposed that the partially degraded VTMoV particle consisting of the stable polypeptide is antigenically similar to the SNMV particle and that these two particles with stable protein subunits are the principal antigens involved in eliciting antibody production in animals. The antigenic and other properties of VTMoV and SNMV are discussed in relation to their taxonomic identity.