Host range and symptomatology of peanut stunt virus (PSV), cucumber mosaic virus (CMV), the Blencowe isolate of tomato aspermy virus (TAV-B) and a chrysanthemum virus (CV-L) varies on selected hosts. In sucrose density-gradient centrifugation each virus has a sedimentation rate of approximately 100s. Purified preparations of PSV and CV-L are homogenous and stable while CMV and TAV-B aggregate and lose infectivity. In acrylamide gel electrophoresis, all the viruses move as a single component with CMV migrating faster than the other three. Formaldehyde-stabilization of PSV and CMV did not increase the titer of immune sera. Classical microprecipitin and gel diffusion tests were unsatisfactory because of nonspecific precipitation and antibodies to normal host antigens. Analysis of incubated homologous and heterologous virus-antibody mixtures by density-gradient centrifugation detected specific precipitation with surface antigenic sites on the virions. In reciprocal tests, CMV and CV-L show no serological relationship. CV-L and TAV-B react reciprocally and are strains. CMV antiserum reacts with TAV-B, but not conversely. PSV reacts reciprocally with the other three viruses, and thus is related at the strain level to all three. Of the viruses compared, PSV appears to be most like the hypothetical parent strain of the CMV group, since it possesses antigenic sites in common to at least three members of the group. / Master of Science