Passive cutaneous anaphylaxis (PCA) reactions were produced in rabbits by antibodies to bovine serum albumin. Two types of antibodies were found, each inducing increased vascular permeability, but by means of different mediation systems. One of these antibodies required the presence of complement, platelets, and neutrophils for the induction of the PCA reaction, which was inhibited by antihistamine. This antibody was heat stable, sedimented in the 7S region, and was found in both fast and slow electrophoretic fractions of rabbit gamma-globulin. Homocytotropic antibody was also detected. The PCA reactions induced by this type of antibody did not require platelets or neutrophils and were not inhibited in rabbits depleted of C3 with cobra venom factor. The lesions were, however, prevented by administration of antihistamine.