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Publisher Summary The chapter discusses an enzyme that has been originally isolated from calf thymus and found to be located primarily in the cytosol. Analysis of endogenous methylated proteins shows that mainly histones are methylated. The enzyme is found in various organs of the rat and is especially elevated in brain, thymus, testis, and spleen. The products of histone methylation by protein methylase I can be identified as N G -mono, N G , N G -di-, and N G , N rG -dimethylarginin. Incorporation of S adenosyl- L [methyl- 14 C] methionine into histone can be measured under conditions (pH 7.2) III favorable for methylation of the guanidino group of arginine residues. Methylation of lysine residues by protein methylase III is negligible at pH 7.2. This chapter discusses the purification procedures where the initial steps are with minor modification. The final enzyme preparation is free of other protein methylases (II and III).