Summary Purified preparations of peanut stunt virus-associated RNA 5 (PARNA 5) and cucumber mosaic virus-associated RNA 5 (CARNA 5) were used to inoculate cowpea protoplasts, alone or together with their respective helper viruses. After incubation of the protoplasts for up to 5 days, total nucleic acid was extracted, denatured, electrophoresed, transferred to nitrocellulose membranes and hybridized to cDNA probes specific for satellite and viral RNA. Satellite replication could be detected only in protoplasts infected with mixtures of satellite and viral RNA, not in those inoculated with satellite RNA alone. These results suggest that the dependence of cucumovirus satellite RNAs on their helpers is on the replication function rather than on the spreading and/or encapsidation functions of the helper virus.