In Artemisia annua, [C-14]arteannuic acid was incorporated into arteannuin-B as well as artemisinin, both in vivo and in a cell free system. Fe2+, 2-oxoglutarate, and peroxidase-H2O2 enhanced the incorporations under in vitro conditions. The results suggest that arteannuic acid might be a common precursor for arteannuin-B and artemisinin synthesis.