Two c-type cytochromes from Chromatium vinosum have been partially purified and characterized. Cytochrome c550, which appears to function as an electron carrier in the cyclic electron transport chain of this photosynthetic purple sulfur bacterium, has a molecular weight of approximately 15,000 and an oxidation-reduction midpoint potential (Em) of +240 mV at pH 7.4. It has (in the reduced form) an alpha band at 550 nm and a beta band at 520 nm. Cytochrome c551 is characterized by absorbance maxima at 354 and 409 nm in the oxidized form and 418, 523, and 551 nm in the reduced form. The reduced cytochrome reacts with CO. Cytochrome c551 is a monomeric protein with a molecular weight of 18,800 +/- 700 and Em = -299 +/- 5 mV (pH independent between pH 6.3 and 8.0). It appears to lack a methionine axial ligand as indicated by the absence of an absorbance band at 695 nm in the oxidized form.