The binding behavior of two heme-propionate side chains in sperm whale myoglobin was evaluated using artificially created hemins, 6-methyl-7-propionate- and 6-propionate-7-methyl-protohemin IX. From the thermodynamic study of the hemin binding to apomyoglobin, it was found that two heme-propionates clearly contribute to the stabilization of the hemin in the protein matrix.