The amino acid sequence of a mouse monoclonal antibody Mab13-1, a catalytic antibody against TCPP (meso-tetrakis(4-carboxyphenyl)porphyrin), was confirmed by mass spectrometric (MS) peptide mapping. The amino-terminal sequence of the heavy chain was established by MS/MS analysis of the isolated N-terminal peptide. The presence of a unique disulfide bond between Cys93H and Cys102H was identified by MS peptide mapping and sequence analysis of an S–S containing peptide. Positions of other disulfide bonds were identified to be conserved. The non-conserved disulfide bridge was found to be resistant as other intra-chain disulfide bonds against reduction under non-denaturing condition, and to be buried inside the molecule. This extra disulfide bond is expected to support antigen-binding by restricting the flexibility of CDR-H3 loop, and it might be favorable for the recognition of a plane antigen, a porphyrin derivative.