A new mechanism is proposed for harman formation which operates, probably secondarily, in Ophiorrhiza and other related Rubiaceous plants. It is based on the observed easy fragmentation of some β-carboline-type monoterpenoid glucoindole alkaloids by enzymatic cleavage of the glucoside bonds. Isolation of two new glucosidic alkaloids, lyalosidic acid and 10-hydroxylyalosidic acid, is also described.