Protein methylase II (S-adensylmethionine:protein-carboxyl-O-methyltransferase, EC.22.214.171.124) which methyl esterifies free carboxyl groups of protein substrate using S-adenosyl-l-methione as the methyl donor, has been purified from human erythrocytes approximately 13 000-fold with a yield of 12%. The purified enzyme was over 95% pure as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A bulk of hemoglobin present in the erythrocyte lysate, which severely limited the use of affinity chromatography for purification, was effectively removed by ammonium sulfate precipitation and by the subsequent salt washing of the precipitates followed by molecular sieve chromatography on Sephadex G-75. This preparation can be further purified by affinity chromatography, in which S-adenosyl-l-homocysteine is covalently linked to Sepharose-4B, followed by Sephadex G-75 chromatography to yield an enzyme with an activity of 27 000 pmol methyl group transferred/mg/min at 37°C. © 1983.