A c3 type cytochrome has been purified from the thermophilic, non-spore-forming, sulfate-reducing bacterium Thermodesulfobacterium commune. The purified protein was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel filtration, and isoelectric focusing. A pI of 6.83 was observed. The molecular weight of the cytochrome was estimated to be ca. 13,000 from both gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hemoprotein exhibited absorption maxima at 530, 408.5, and 351 nm in the oxidized form and 551.5 (alpha band), 522.5 (beta band), and 418.5 nm (gamma band) in the reduced form. The extinction coefficients of T. commune cytochrome c3 were 130,000, 74,120, and 975,000 M-1 cm-1 at 551.5, 522.5, and 418.5 nm, respectively. It contains four hemes per molecule, on the basis of both the iron estimation and the extinction coefficient value of its pyridine hemochrome. The amino acid composition showed the presence of eight cysteine residues involved in heme binding. T. commune cytochrome c3 had low threonine, serine, and glycine contents and high glutamic acid and hydrophobic residue contents. The electrochemical study of T. commune cytochrome c3 by cyclic voltammetry and differential pulse polarography has shown that the cytochrome system behaves like a reversible system. Four redox potential values at Eh1 = -0.140 +/- 0.010 V, Eh2 = Eh3 = Eh4 = -0.280 +/- 0.010 V have been determined. T. commune cytochrome c3, which acts as the physiological electron carrier of hydrogenase, is similar in most respects to the multiheme low-potential cytochrome c3 which is characteristic of the genus Desulfovibrio.