The nucleocapsid (NP) of wheat rosette stunt virus (WRSV) was used to separate L, NS and Nproteins. By two-step dissociation of the NP particles and subsequent ultracentrifugation through a gly-cerol cushion, four viral fractions were obtained: L protein, NS-N-RNA complex, NS protein and N-RNA complex. All these fractions did not demonstrate RNA polymerase activity in vitro when assayedindividually. After various recombinations of these fractions, we found that the RNA polymerase activitywas reconstituted only when the L, NS and N-RNA complex were all present in the assay mixture. Sincethe dissociable L and NS proteins were both required for the RNA synthesis in vitro, we presume thatthey probably constitute together the RNA polymerase complex. The fact that N-RNA complex could notbe replaced by phenol extracted RNA as template for the in vitro transcription, and the tight associationof N protein with the viral RNA suggests that the N protein associated with the RNA may function asa factor which makes t By two-step dissociation of the NP particles and subsequent ultracentrifugation through a gly-cerol cushion, four viral fractions were obtained: L protein (WRSV) was used to separate L, NS and Nproteins. , NS-N-RNA complex, NS protein and N-RNA complex. All these fractions did not demonstrate RNA polymerase activity in vitro when assayed divide. After various recombinations of these fractions, we found that the RNA polymerase activity was reconstituted only when the L, Both NS and N-RNA complex were all present in the assay mixture. Since the dissociable L and NS proteins were both required for the RNA synthesis in vitro, we presume that the can probably together the RNA polymerase complex. The fact that N-RNA complex could notbe replaced by phenol extracted RNA as template for the in vitro transcription, and the tight association of N protein with the viral RNA suggests that the N protein associated with the RNA may function asa fa ctor which makes t