An S-adenosyl-L-methionine:mRNA(nucleoside-2'-)-methyltransferase, one of at least three activities required for the 5'-terminal modification of mRNA, has been purified from vaccinia virus particles. Employing brome mosaic virus RNA ending in m7G(5')pppG- as substrate, a simple DEAE-cellulose filter assay measuring the incorporation of methyl groups from S-adenosyl[methyl-3H]methionine to position 2' of the penultimate nucleoside was devised. Starting from disrupted vaccinia virus cores, a 350-fold enzyme purification was achieved by successive chromatography on columns of DEAE-cellulose, CM-Sephadex, and APP-agarose. Analysis of the isolated enzyme by sodium dodecyl sulfate-polyacrylamide discontinuous gel electrophoresis revealed a single polypeptide with a molecular weight of 38,000. Similar molecular weights were obtained by sucrose gradient centrifugation and gel filtration of the native methyltransferase. The isoelectric point of the purified enzyme occurs at pH 8.4.