Binding of auxin-type herbicides to maize membrane-bound auxin binding protein was compared with their physiological activities.There was a good correlation between the binding and the growth promoting activity of the herbicides, but the ethylene production or the inhibition of shoot growth did not completely correlate with the binding. Benazolin[4-chloro-2-oxobenzothiazolin-3-yl-acetic acid] did not bind to maize auxin binding protein, but it stimulated ethylene production.Quinclorac [3, 7-dichloro-8-quinolinecarboxylic acid] did not bind to maize auxin binding protein, but it strongly inhibited maize shoot growth. These results may suggest that the stimulation of ethylene production and the growth inhibition by auxin-type herbicides are not triggered by their binding to the putative maize membrane-bound auxin receptor.