GTP:RNA guanylyltransferase, the enzyme which catalyzes the guanylylation of the 5' termini of viral mRNAs, has been isolated and purified approximately 10,000-fold from cores of vaccinia virus. S-adenosyl-methionine:mRNA (guanine-7)-methyltransferase copurified with guanylyltransferase activity through chromatography on DNA agarose, phosphocellulose, and centrifugation in glycerol gradients, suggesting that the two activities are closely associated. The molecular weight of native guanylyltransferase- and 7-methyltransferase-associated activities was approximately 120,000 as determined by glycerol gradient centrifugation. Guanylytransferase purified by electrophoresis on polyacrylamide gels at pH 4.5 lacked 7-methyltransferase activity. Analysis by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels of electrophoretically purified native guanylyltransferase showed the presence of one major band of polypeptide which had a molecular weight of approximately 59,000.