text: The development of the classic theory of drug antagonism by Gaddum, Schild and Arunlakshana built on the work of Langley, Hill and Clark. Gaddum described a model for the competitive binding of two ligands to the same receptor in short communication to The Physiological Society in 1937. The description referred only to binding, it was not immediately useful for the analysis of experimental measurements of the effects of antagonists on the response to agonists. It was Heinz Otto Schild who made measurement of the equilibrium constant for the binding of an antagonist possible.  He developed the Schild equation to determine a dose ratio, a measure of the potency of a drug. In Schild regression, the change in the dose ratio, the ratio of the EC of an agonist alone compared to the EC in the presence of a competitive antagonist as determined on a dose response curve used to determine the affinity of an antagonist for its receptor.