Five preparations of purified antigens from S. aureus (A-l, A-2, A-3, B and C) were studied for their effect on the reactivity of the rabbit skin to epinephrine and their stimulating action on isolated ileum from normal guinea-pigs. In addition, a study in agar gel with the staphylococcal antigens and sera from normal guinea-pigs and rabbits was performed. Fractions A-2 and A-3, but not A-l, B and C, produced haemorrhagic lesions in the epinephrine-enhancing test. Fractions A-l, A-3 and B, but not A-2 and C, caused isolated guinea-pig ileum to contract. Two types of responses were recorded, the one in tests with fraction A-l and fraction B, the other in assays with fraction A-3. Precipitins against fraction A-l, which contains protein A, were invariable demonstrated in the guinea-pig sera (131 sera tested), but no other precipitins were found, either in the guinea-pig sera or in the rabbit sera. Anaphylaxis to protein A was suggested as a possible mechanism in the reaction of the guinea-pig ileum to fraction A-l.