Each plant hormone elicits a set of characteristic physiological and biochemical responses. The primary stimulus initiating the sequence of events leading to such responses is the interaction of the hormone with its receptor protein. In general, receptors may be either membrane-associated or soluble (or solubilized) and this chapter deals with one binding technique suitable for each class. Some alternatives are briefly mentioned in Subheading 4, but for a comprehensive treatment of data acquisition and analysis, Hulme (1) is strongly recommended, together with the chapter by Klotz in ref. 2 on equilibrium dialysis.