Reverse passive cutaneous anaphylaxis experiments were performed in guinea pigs with rabbit gamma globulin and human gamma globulin as antigens and horse, sheep or hen sera containing antibodies against these antigens. These antibodies were unable to directly sensitize the guinea-pig skin but when the reverse technique was used, namely, the antigen (gamma globulin) was injected before the antibody, characteristic anaphylactic reactions were obtained. It is concluded that, if a suitable antigen is used, which can be fixed to the host by the reverse technique, sera from species which cannot directly sensitize the guinea pig can nevertheless give typical anaphylactic reactions.