While the potential dangers of immediate and delayed reactions following parenteral administration of horse serum are well recognized by the medical profession, the possibility that serum sickness may result in serious complications is not as well known. One of the most serious sequels following prophylactic and therapeutic injection is horse serum neuritis. Serum sickness, a delayed type of reaction, occurs from six to fourteen days after the injection of horse serum. It is in no way dependent on atopic, hereditary sensitiveness or anaphylaxis (artificial sensitization by previous injections that at times causes immediate, life-endangering reactions). Serum sickness probably would occur in all cases if enough serum were given. Local itching, swelling, generalized urticaria, fever, enlargement of lymph glands, polyarthritis, general malaise, leukopenia, albuminuria, fall of blood pressure and decreased coagulability of the blood make up the complete clinical picture. The severity of the reaction varies greatly. The cause is in