Haptens are defined as substances which react specifically with antibody, but cannot provoke allergic reactions without being attached to a macromolecular carrier by covalent bonds, nor carrying two or more identical antigenic determinants. Having examined the biological activity of glycine ester, aspirin and their derivatives by PCA and Schultz-Dale reaction, it was confirmed that PCA was obtained using some univalent haptens containing only a single haptenic group, and that these haptens acquired this ability by reversible binding to serum albumin. Using as antigen a mixture of the dihaptenic substance containing two different determinant groups and the specific antibody globulin to one determinant, positive Schultz-Dale reaction was obtained on the intestinal strips sensitized with the specific antibody to another determinant. It was found that these haptens were incapable of desensitizing the sensitized animals, nor provoking anamnestic response.