The macroscopic and microscopic redox potentials of cytochrome c(3) from Desulfovibrio vulgaris Hildenborough (DvH) were estimated by the combined use of H-1 NMR and differential pulse polarography at two ionic strengths. The redox potentials obtained were compared with those of Desulfovibrio vulgaris Miyazaki F (DvMF) to see the effect of amino acid substitution in the 13 residues. Although the general tendencies are similar to each other, there are distinct differences. The average redox potentials of DvH cytochrome c(3) were lower than those of DvMF by about 34 mV. The difference in the microscopic redox potentials was significant for hemes 1 and 3. The cooperative interaction between hemes 1 and 2 was larger in DvH cytochrome c(3) than in DvMF cytochrome c(3) and the former was ionic strength dependent. The microscopic redox potentials of DvH cytochrome c(3) increased with an increase of ionic strength.