We have previously shown that the vesicular stomatitis virus (VSV) host range mutant, hr 1, is completely defective for the mRNA methyltransferase activities, but can synthesize full-length, unmethylated mRNAs in vitro [S. M. Horikami and S. A. Moyer (1982). Proc. Natl. Acad. Sci. USA 79, 7694-7698] and in vivo [S. M. Horikami, F. De Ferra, and S. A. Moyer (1984). Virology 138, 1-15]. Here we have used the hr 1 mutant to identify the viral protein which possesses the methyltransferase activities. The wild-type VSV L and NS proteins, subunits of the viral RNA polymerase, were separately purified and added to high salt dissociated mutant hr 1 nucleocapsids for in vitro transcription reactions. The results show that the purified wild-type L protein, but not the NS protein, restores methylation and thus possesses the viral mRNA methyltransferase activities.