The commercially used cell suspension culture of Coptis japonica Makino (Ranunculaceae) incorporates [β-13C]tyrosine into both halves of the berberine molecule (isoquinoline and benzyl portion) while L-[3′-18O]DOPA only labels the isoquinoline portion of this molecule. This labelling pattern indicates that different C6-C2 units derived from tyrosine form this molecule. This result and the incorporation of (S)-[1-13C] norcoclaurine into berberine demonstrates that the biosynthesis of protoberberine alkaloids in Coptis involves the norcoclaurine but not the norlaudanosoline pathway.