Transgenic Nicotiana tabacum plants expressing RNA sequences of the tomato spotted wilt virus NS(M) gene, which encodes the putative viral movement protein, were found to be highly resistant to infection with the virus. Expression of untranslatable as well as anti-sense RNA of the NS(M) gene resulted in resistance levels as high as those in plants expressing translatable RNA sequences. For all three types of transgenic plants resistance levels of up to 100% were reached in the S2 progeny. These results indicate that the resistance mediated by the NS(M) gene is accomplished by expression of transcripts rather than protein in transgenic plants, similar to previously observed N gene-mediated resistance. Protoplast inoculations revealed that resistant plants expressing NS(M) are, in contrast to N transgenic resistant plants, not resistant at the cellular level. This suggests the RNA-mediated resistance mechanism against TSWV targets viral mRNAs rather than the viral genome.