Posttranslational modification of proteins by the enzyme protein carboxyl methyltransferase (PCM) has been associated with a variety of cellular functions. A prerequisite for the understanding of cellular mechanisms associated with PCM is the characterization of purified PCMs from different tissues. We describe here the purification and characterization of PCM from the electric organ of Torpedo ocellata. The enzyme was purified to homogeneity by ion-exchange chromatography and ammonium sulfate precipitation, followed by chromatography on Sephadex G-100 and hydroxylapatite columns. When visualized by silver staining, the 700-fold-purified PCM exhibited a single band on sodium dodecyl sulfate-polyacrylamide gels, corresponding to a polypeptide of Mr29 000. The molecular weight of the nondenatured enzyme (as determined by rechromatography on Sephadex G-100 column) was also 29000, suggesting that the enzyme is a monomer. Two isoelectric forms of PCM (p/ = 6.1 and pI = 6.4) were detected in the purified enzyme preparation. The enzyme methylates various exogenous and endogenous proteins, including the acetylcholine receptor. Of the four different polypeptides of the acetylcholine receptor, the γ and β polypeptides were selectively methylated by the purified PCM. Purified Torpedo PCM is highly sensitive to sulfhydryl reagents. The competitive inhibitor of PCM S-adenosyl-L-homocysteine (AdoHcy) protected the enzyme from inactivation by sulfhydryl reagents, suggesting the existence of a cysteine residue at the active site of the enzyme. The purified PCM has a low affinity toward DEAE-cellulose and toward AdoHcy-agarose. This property, as well as the relatively high molecular weight and the marked sensitivity to sulfhydryl reagents, distinguishes between the electric organ PCM and analogous enzymes of mammalian tissues.