In the course of exploring the possibility of employing crystallized bovine albumin (Cohn) as a blood substitute, instances of delayed serum sickness characterized by myalgia, arthralgia, urticaria, petechial rash and fever occurring ten to twenty-one days after injection was noted in approximately 8 per cent of the patients injected. The usual therapeutic agents such as epinephrine, ephedrine, codeine and calcium gluconate, although helpful in the mild cases, afforded no relief in the more severe reactions. The generalized myalgia and arthralgia in some cases were especially severe, and it was primarily to relieve distressing pain that intravenous procaine was given. However, the subsidence of all the manifestations of serum sickness was a pleasant surprise and occurred with such frequency as to encourage its employment in all subsequent instances of serum sickness. Furthermore, we have attempted to explore its utility in the management of other states of hypersensitivity. A series of preliminary