To investigate the biosynthetic pathway leading to artemisinin, the putative precursors, arteannuic acid and 11,12-dihydroarteannuic acid, were incubated in a cell-free system. For the experiment with dihydroarteannuic acid, artemisinin was generated in tumor homogenate. These results showed that dihydroarteannuic acid could be converted to artemisinin enzymatically. However, the experimental condition failed to convert arteannuic acid into 11,12-dihydroarteannuic acid(Received February 20, 1992, accepted March 31, 1992).