Derivative UV spectroscopic data show that the plant growth substances 6-(γ,γ-Dimethylallylamino)purine (N6-(delta 2-isopentenyl) adenine, i6Ade)and indolylacetic acid (IAA) can bind to the yeast alcohol dehydrogenase (ADH) and affect coenzyme-enzyme binding. At fixed ethanol concentrations (27.8 and 111.1 mM) and varying NAD ⁺ concentrations (0.033-2 mM), as well as at fixed levels of coenzyme (0.67 and 2 mM), and at varying concentrations of ethanol (1.4-111.1 mM), the rate of ethanol oxidation is significantly inhibited by i6Ade and IAA. The kinetics of the ADH reaction is affected by two inhibition constants (K _i and K _i ') which correspond to the dissociation constants of complexes EI and ESI, respectively. For i6Ade the K _i =0.52±0.06 mM and K _i '=0.74±0.07 mM, and for IAA the K _i =0.88±0.03 mM and K _i '=0.99±0.02 mM.