In callus and cell suspension cultures ofArtemisia annuaminute amounts of artemisinin were found. In these cultures, a high peroxidase activity was measured, intracellularly and, especially, in the medium.In vitro, artemisinin rapidly decomposed when incubated with a cell homogenate ofA. annuaor with spent culture medium, as well as in a solution of commercially obtained horseradish peroxidase (EC 1.11.1.7). In contrast, the peroxidase activity in the intact plant, containing considerable amounts of artemisinin, was low. We suggest that the high peroxidase activity inA. annuacell cultures contributes to their very low artemisinin contents.