The binding activity of purified alpha-galactoside receptor proteins from a number of plant species decreases when the protein concentration is increased from 2 ng/ml to 100 micrograms/ml. 2. The apparent loss of binding activity at high protein concentrations corresonds to the formation of high molecular weight multimers. 3. Raffinose and melibiose cause a ligant-dependent increase in binding activity and a corresponding decrease in the relative abundance of multimers at any give protein concentration. 4. The self-inhibition of binding activity at high protein concentrations arises from a competition between ligant binding by oligomers and self-association of these oligomers into multimeric species which have little or no binding acitivity.
