Inhibitory effect of alantolactone and isoalantolactone was shown in Avena straight growth test and in the formation of adventitious root in Phaseolus seedling. However, di-, and tetrahydroalantolactones were given no effect on the elongation and the rooting. Inhibitory effect of alantolactone could partly be removed by cysteine, cystine, and reduced glutathione. The plant materials were made less sensitive to alantolactone by the pretreatment of cysteine, but cysteine supplied after the treatment of alantolactone brought about no effect on the action of alantolactone. A new spot was shown on TLC plate from the mixture of alantolactone and cysteine, indicating that alantolactone can be inactivated by cysteine, not cystine, without any biological processes.