The intensity of anaphylactic shock was lower in C3H mice carrying a methylcholanthrene-induced tumour (McC3) than in their normal counterparts when immunized with ovalbumin and challenged i.v. after 14 days. This tumour-associated inhibitory effect on active systemic anaphylaxis was exerted mainly on events occurring after homocytotropic antibody synthesis because the serum titres of these antibodies were comparable in normal and tumour-bearing animals. In addition, passive systemic anaphylactic reactions were suppressed in animals carrying the tumour and the sensitivity of these animals to challenge with histamine and serotonin mixtures was also reduced. The presence of a growing McC3 tumour did not, however, diminish the amine-sensitizing effect of treatment with Bordetella pertussis vaccine. The McC3 tumour inhibited the generation of passive cutaneous anaphylactic reactions, an effect that was also exerted by a tumour extract, particularly when administered to the recipients shortly before antigen challenge. Thus immediate hypersensitivity reactions, like a variety of other immunological processes, can be inhibited by tumour products which by compromising the immune status of the host might permit tumour growth. The nature of the inhibiting factor is unknown, except that it is probably not the amine-degrading enzyme histaminase. In addition, which it is uncertain whether the inhibitory effect is exerted directly or indirectly, the possible importance of prostaglandins in the phenomenon is discussed.