Hybridomas were established that secreted antibodies specific for soybean mosaic virus (SMV), lettuce mosaic virus (LMV), maize dwarf mosaic virus A (MDMV-A), or maize dwarf mosaic virus B (MDMV-B). Virus-specific monoclonal antibody immobilized on protein A-Sepharose was active in binding virus particles from solution. Monoclonal antibodies against SMV were used in a double antibody sandwich radioimmunoassay (RIA). An RIA using one type of monoclonal antibody lacked sensitivity because limited antigenic sites were available. Results obtained using two monoclonal antibodies that bound to different antigenic sites were comparable to those obtained using polyclonal antibodies. Inhibition RIA assays of SMV, LMV, MDMV-A, and MDMV-B were sensitive to 25 ng when single monoclonal antibodies were used.