The investigation of the effects of simultaneous intravenous injections of antigen and antibody occupies one of the most interesting and important chapters in anaphylaxis. About ten years ago Otto (1) first showed that passive sensitization is invariably accompanied by a latent period of several hours duration. He interpreted the latent interval simply as the period of time required by the cells to appropriate the introduced antibody, and drew the fundamental inference that the anaphylactic response could not take place except through the mediation of cellular antibody. This theory, which is quite in harmony with Besredka's views, was at first very generally accepted. The discovery of anaphylatoxin, however, gave rise to a rival and altogether irreconcilable theory of anaphylaxis. When it was found that the incubation of antigen with immune serum in the test tube gave rise to violent poisons, it seemed natural to infer that the same poisons would be produced when antigen was injected into the sensitized animals.