Summary: Cutaneous and systemic anaphylaxis-like reactions as well as hemorrhagic reactions were produced in nonimmunized guinea pigs by injection of a bacterial cell wall protein (protein A) from Staphylococcus aureus. These phenomena are believed to be mediated by a reaction between protein A and the Fc region of γ-globulin. Protein A is thus able to produce in normal guinea pigs reactions indistinguishable from immediate hypersensitivity reactions.