A proteinaceous inhibitor of auxin-induced ethylene synthesis by mungbean hypocotyls was purified from mungbean seeds by chromatography on CM-Sephadex C-50 and gel filtration on Sephadex G-200. The molecular weight was estimated to be about 89, 000 by gel filtration on Sephadex G-200. The inhibitory protein consisted of two subunits with molecular weights of about 58, 000 and 31, 000.