A new bisbenzylisoquinoline alkaloid, stebisimine, has been isolated from Stephania japonica Miers and shown by physical methods and chemical degradation to be N-nor-1,2-dehydro-epistephanine. Tracer experiments have shown that half of the epistephanine molecule is derived exclusively from (-)-N-methylcoclaurine but not from its enantiomer. This confirms the absolute configuration of the alkaloid and shows that racemisation of the (+)-enantiomer is unimportant in this plant.