Dose-response curves are often used in the study of the interaction of hormones and receptors. From these plots, IC50 or EC50 values are calculated. In these pharmacological assays it is implicitly assumed that a single receptor predominates in a tissue. In this paper the interaction of a ligand with two receptors is studied from a theoretical point of view. It is assumed that the responses mediated by these receptors are qualitatively or quantitatively different. The theoretical direct and Scatchard plots display a high variety of shapes depending upon the difference in potency of the effect of the drug acting in both receptors and upon the magnitude and sign of the individual response. When dose-response curves taken from the literature have been transformed into direct or Scatchard plots new information has become available. With respect to this, it is shown that agonists of purinergic receptors seem to interact with two different populations of receptors. We claim that carefully designed experiments must provide valuable information concerning the number of subtypes of receptors present in a given system and the kind of response mediated by them.