Peroxidase active preparations from three Ancistrocladus species and from Triphyophyllum peltatum have been partially purified. They catalyze the oxidative dimerization of korupensamines A and B to michellamines A and C, respectively, as well as the mixed coupling to michellamines A, B, and C. All of these enzymes consist of single polypeptides of approximately 65 kDa with peroxidase activity as monomers. They were characterized as soluble proteins predominantly localized in the leaf phloem of all species examined. Comparative studies with various naphthol precursors revealed an unexpected substrate specificity. Only korupensamines were dimerized by the enzymes, while other monomers, even if closely related, were not accepted as substrates. The coupling reactions described here represent the first direct synthesis of michellamines from korupensamines without previous protection of these precursors.