1 The bronchospasm induced in the guinea-pig by the injection of Forssman antiserum was biphasic in nature in both the sublethal and the lethal reaction. 2 The development of both phases of the bronchospasm in the sublethal reaction was dependent upon the presence of the intact complement system and circulating platelets. In the lethal reaction the phase II bronchospasm did not appear to depend on these factors. 3 The compounds used in this study inhibited phase I bronchospasm of the sublethal reaction in the order, methysergide > indomethacin > aspirin = sulphinpyrazone and phase II in the order, indo-methacin > sulphinpyrazone > aspirin. Methysergide was inactive. 4 Aspirin, indomethacin and sodium salicylate all prevented the inhibitory action of sulphinpyrazone in reducing the phase II bronchospasm of the sublethal reaction in the order, indomethacin > sodium salicylate > aspirin, when the drugs were administered prior to sulphinpyrazone. 5 The inhibitory action of aspirin on the sulphinpyrazone effect could be prevented by administering sulphinpyrazone before aspirin. All drug-induced inhibitions of sulphinpyrazone by aspirin, indomethacin and sodium salicylate were dose-dependent.