The catalytic antibody which catalyzes insertion of a cupric ion into mesoporphyrin bound ferric N-methyl mesoporhyrin and ferric mesoporhyrin to yield the respective complexes. The binding affinity of cyanide to the ferric iron in the complexes was compared with that of free ferric porphyrins. The results indicate that the one side of the porphyrin plane in the complex is thoroughly exposed to surrounding solvents and the other side interacts with the protein molecule. The cyanide binding suggests the specific orientation of distorted porphyrin at the binding site of the antibody.