The term 'gene silencing' refers to transcriptional and post-transcriptional control of gene expression. Related processes are found across kingdoms in plants and animals. We intended to test whether particular RNA constituents of a silenced plant can induce silencing in an animal. We generated Nicotiana benthamiana lines that expressed green fluorescent protein (GFP) from a transgene. Plants in which GFP expression was spontaneously silenced showed siRNAs characteristic of post-transcriptional gene silencing (PTGS). RNA extracts prepared from silenced plants were injected into a GFP-expressing strain of Caenorhabditis elegans, where they induced RNA interference (RNAi). Extracts from non-silenced plants were inactive. This directly demonstrates a relationship and a mechanistic link between PTGS and RNAi. Controls confirmed that the silencing agent was an RNA. Size fractionation on denaturing gels revealed that an RNA of approximately 85 nt was most active in inducing silencing in the worm. Northern blot analysis of the region in question did not detect a prominent GFP-specific RNA of sense or antisense polarity, indicating that the RNA species which induced silencing was present only in low concentration or did not hybridize due to formation of an intramolecular double strand. In view of its high activity, it is possible that this agent is responsible for the systemic spread of silencing in plants and it might represent the aberrant RNA, a previously postulated inducer of silencing.