Artemisinin production by hairy roots of Artemisia annua L. was increased 6-fold to 1.8 mu g mg(-1) dry wt over 6 days by adding 150 mg chitosan l(-1). The increase was dose-dependent. Similar treatment of hairy roots with methyl jasmonate (0.2 mM) or yeast extract (2 mg ml(-1)) increased artemisinin production to 1.5 and 0.9 mu g mg(-1) dry wt, respectively.