Complex formation of porphyrin derivatives with zwitterionic poly(Glu-Leu-Lys-Leu) was investigated as a protein-porphyrin complex model. The beta -sheet structure of poly(Glu-Leu-Lys-Leu) interacted with a 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (TPPS) diacid form which has positive charge in the porphyrin center. On the other hand, no interaction of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP) or a TPPS free form with the polypeptide was observed. The positive charge in the porphyrin center for the diacid species may play an important role in the complex formation. The TPPS diacid species was aggregated by the addition of the polypeptide and/or NaCl; at least three kinds of TPPS aggregates were observed.