Beet necrotic yellow vein virus (BNYVV) inocula with different RNA compositions were prepared from infectious transcripts of RNAs 3 and 4 and the Rg 1 isolate, which has a genome consisting only of RNAs 1 and 2. The recombinant viruses were inoculated on 6- to 8-day-old sugarbeet seedlings by 'vortexing'. Inocula containing RNAs 1 and 2 or 1, 2 and 4 produced some growth reduction, but the most dramatic effects, with yield reductions of about 95% in a highly susceptible variety, were seen when RNA 3 was also present in the inoculum. Under these conditions the side roots were brown and brittle and often deteriorated, but 'root beardedness' was not observed. This might be due to the fact that our experiments were done in the absence of Polymyxa betae. Alternatively, the heavy inoculation at a very young age may either have weakened the plants to such an extent that extensive root proliferation was impaired or it may have led to rapid deterioration of the proliferating rootlets, which would therefore be lost prior to or during removal of the tap roots from the soil. In the presence of RNA 3 the virus concentrations in tap roots were markedly increased suggesting that this RNA facilitates the multiplication and/or spread of the virus in root tissues.