Raspberry ringspot virus (R/1:2.4/43 + 1.4/30 (or 2 × 1.4/46):S/S:S/Ne, nepovirus group) is a multicomponent plant virus with two sizes of RNA having mol. wt. of 2.4 × 106 (RNA-1) and 1.4 × 106 (RNA-2) (Murant et al. 1972). When obtained by centrifuging in sucrose density gradients or by electrophoresis in polyacrylamide gels, preparations of RNA-2 mostly did not infect the local lesion assay host, Chenopodium amaranticolor Coste & Reyn., and those of RNA-1 were only moderately infective, whereas mixtures of the two kinds of preparation were very infective (Harrison, Murant & Mayo, 1972). No such increase in infectivity occurred when one RNA species from raspberry ringspot virus was mixed with the other from tobacco ringspot virus (another nepovirus), or when either of the RNA species from raspberry ringspot virus was u.v. irradiated before use. A possible explanation of these results is that each RNA species carries genetic information not carried by the other.