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A technique is described for measuring objectively and quantitatively the reaction of sensitized guinea pigs when exposed to an aerosol of specific antigen. The principle involves registration by semi-automatic means of the number of coughs produced in animals passively sensitized with known amounts of antibody. The number of coughs is shown to be linearly related to log dose of antibody within a limited range and a dose-response curve is presented. The cough produced by this procedure is not inhibited by the antitussive drugs, codeine and propadrine, but can be inhibited by anti-allergic agents, such as cortisone and an antihistaminic drug. It is also inhibited by narcotine. The last is the only compound so far tested which suppresses both the cough produced by this procedure and that produced by a simple irritant. The action of cortisone and the antihistaminic drug, pyrilamine, is shown to be synergistic. A small dose of pyrilamine in animals pretreated with cortisone gives a degree of inhibition which cannot be obtained by increasing the dose of pyrilamine in animals not treated with cortisone.