Feeding experiments in which 14C-tryptophan was administered to cell cultures of Ailanthus altissima showed that the biosynthetic sequence of the produced alkaloids is: tryptophan (1)↦β-carboline-1-propionic acid (2)↦4,5-dihydrocanthin-6-one (3)↦canthin-6-one (4)↦1-hydroxy-canthin-6-one (5)↦1-methoxy-canthm-6-one (6)↦1-methoxy-canthin-6-one-3-oxide (8). 2-Hydroxy-canthin-6-one (9), 4-hydroxy-canthin-6-one (11), and 5-hydroxycanthin-6-one (10) derive from 4, but their methoxyderivatives were not detected in our cultures.Canthin-6-one-3-oxide (7) also derives from 4 but is not further transformed. 14C-Tryptamine was ineffective in labelling the alkaloids. Except for canthin-6-one (4) and 1-methoxy-canthin-6-one (6), none of the above alkaloids has ever been described in plant cell cultures, and 4,5-dihydrocanthin-6-one (3), 2-hydroxy-canthin-6-one (9), and 4-hydroxy-canthin-6-one (11) have never actually been found in nature. © 1986, American Chemical Society. All rights reserved.