Roots, stems, petioles and leaf blades of mature plants of P. hysterophorus and callus cultures derived from explants of these organs from aseptically grown seedlings were screened for the presence of sesquiterpene lactones. Parthenin was not detected in roots but was present in callus cultures from all sources. Coronopilin was detected only in petioles and leaf blades and callus cultures derived from these organs. Synthesis of parthenin and coronopilin apparently is not limited to the glandular trichomes.