A protein which reversibly inhibits auxin-induced ethylene synthesis has been isolated and purified from hypocotyls of etiolated mungbean (Phaseolus aureus Roxb.) seedlings. The molecular weight of the inhibitor was estimated to be 112 000 by gel filtration and polyacrylamide gel-electrophoresis. When treated with sodium dodecylsulfate, the inhibitor gave on polyacrylamide gel-electrophoresis a single band corresponding to a molecular weight of 56 000, indicating that it consisted of two subunits with identical molecular weight. The inhibitor does not degrade nor bind indole-3-acetic acid, and has no peroxidase activity.