Cucumber mosaic virus (CMV) satellite RNA (Sat-RNA, D. W. Mossop and R. I. B. Francki, 1978, Virology86, 562-566) is similar in many of its physical and biological properties to CMV associated RNA 5 (CARNA 5) described by Kaper and Tousignant (1977, Virology85, 323-327). However, CARNA 5, unlike Sat-RNA, causes a serious necrotic disease of tomato. Sat-RNA when inoculated together with various CMV or tomato aspermy virus strains not only failed to increase the severity of symptoms in infected tomato plants, but ameliorated them in some instances. Comparisons of the two RNAs by hybridization analysis using 32P-labelled complementary DNA probes, indicate that they have partial nucleotide sequence homology. It is suggested that the difference in their primary structure is reflected in their biological properties.