A multiplicity of untoward sequelae have been observed in patients treated with immune serum. Serum sickness, which is by far the most common manifestation of hypersensitiveness to horse serum, is usually of only short duration and is more disconcerting than dangerous to the patient. The common symptomatology includes fever, urticaria, erythema or edema, lymphadenopathy, arthralgia, smothering sensations, headache, nausea and vomiting. Occasionally there are more serious and lasting manifestations, such as peripheral neuritis, hepatitis with jaundice, epididymitis and orchitis or unusual lymphadenopathy. Rarely, death has been reported from true anaphylaxis. Even more infrequent are reports of a type of local anaphylactic reaction called the phenomenon of Arthus, which consists essentially of necrosis at the site of serum injection. The following cases are reported because they represent this unusual type of local tissue hypersensitiveness and because they demonstrate how very readily such a condition may be acquired. REPORT OF CASES Case