Four isoforms of strictosidine synthase (EC 4.3.3.2), which catalyses the stereospecific condensation of secologanin and tryptamine, were purified to homogeneity from a suspension culture of Cinchona robusta. The M(r)s are 35 000, 33 000, 35 000 and 33 000, and the isoelectric points are 6.5, 6.5, 7.5 and 7.5, respectively. The K(m) values of the four proteins for tryptamine are in the micromolar range. The enzymes are inhibited by the product strictosidine and by quinoline alkaloids. The K(i) of quinine for one of the main isoforms is ca 7 muM. No substrate inhibition was observed. In contrast with strictosidine synthase from Catharanthus roseus (Apocynaceae), all four isoforms of Cinchona robusta accepted 5-methoxytryptamine as a substrate. The enzyme does not catalyse the condensation of corynantheal with tryptamine derivatives. All four isoforms are glycosylated.