Human sera containing precipitins to cow’s milk proteins or to tetanus toxoid failed to sensitize nonhuman primates for cutaneous or systemic anaphylaxis. Human reaginic serum, tested in the same experimental animals, successfully sensitized nonhuman primates for both cutaneous and systemic anaphylaxis. The human precipitin-containing sera passively sensitized guinea pigs for both cutaneous and systemic anaphylaxis, whereas the human reaginic sera failed to do so. The precipitin activity of the sera was demonstrated by immunoelectrophoretic separation to reside in a scrum fraction corresponding to the immunoglobulin class IgG. © 1966 S. Karger AG, Basel.