The strength of binding of a fungal elicitor derived from Rhizoctonia solani to Agrobacterium-transformed root cultures of Hyoscyamus muticus is assessed. Two techniques are used to estimate the equilibrium binding constant : (1) a dose response, which involves the measurement of sesquiterpene formation in the presence of different amounts of elicitor, and (2) elicitor-contacting experiments in which the root tissue is exposed to the elicitor for a short period of time and then the unbound elicitor concentration is determined by a bioassay. The equilibrium binding constants estimated by these two techniques were on the same order of magnitude. The value predicted from the dose response technique was 6.44 (mmol of glucose equiv/L of medium)(-1), while that obtained from elicitor contact was 1.89 (mmol of glucose equiv/L of medium)(-1) The higher binding constant measured by dose response may result from feedback inhibition of secondary metabolite formation at higher elicitor dosages. The magnitude of the binding constant indicates that elicitor dosage is dependent upon both the amount of tissue in the reactor and the amount of medium present to dilute the added elicitor. Since proposed reactor configurations for plant root culture vary from 200 to 2000 g of tissue per liter of medium, elicitor dosage requirements will vary significantly depending upon the reactor operational conditions. This influence of elicitor binding on reactor design and operation is discussed.