Based on quantitative dose-effect studies with the transmitter substance acetylcholine, Clark suggested the so-called receptor occupancy theory. According to this theory the intensity of the pharmacological effect of a substance was directly proportional to the number of cell receptors occupied by the substance [21]. Gaddum, working on the dose-effect relations of adrenalin and ergotamine elucidated the competitive antagonism between two substances at receptors and introduced the notion of receptor blockage by the antagonistic substance [22]. Clark publicly attacked Straub over his physical potential-poison theory, saying at a meeting of the Royal Society in London in 1936 that it assumed processes that were unknown in physical chemistry [23].