1. Our experiments show that an intravenous injection of antigen immediately after an intravenous injection of antiserum in the guinea-pig were followed by:
(a) Acute shock and death within 5 min. The signs and post-mortem appearances were indistinguishable from acute anaphylactic shock as typically seen in the guinea-pig. Some control animals injected with antiserum only died in a way similar to those which received both antiserum and antigen. These are discussed in detail in the body of the paper.
(b) Delayed shock and death some hours later. The post-mortem appearances were those of gastro-intestinal congestion and haemorrhage resembling the changes seen in dogs dying of anaphylactic shock. Such changes were never seen in the control animals.
(c) Recovery. Practically all the animals which recovered had symptoms of respiratory embarrassment immediately following the injections of anti-serum and antigen and many had later symptoms of abdominal shock. The animals which were given an injection of antiserum only rarely had any symptoms and never abdominal symptoms.
2. It is necessary to test the antisera used by control inoculation since some antisera are toxic.