Limited chymotryptic digestion of chicken-liver sulfite oxidase destroys its ability to oxidize sulfite. From the digest can be isolated a heme-binding fragment of molecular weight about 11 000. Its purification is described, as well as its characterization by a number of methods (absorption spectroscopy, circular dichroism, electrophoretic mobility, immunochemical reactivity, amino acid analysis). The heme spectrum shows no detectable difference with that of the native enzyme. The N-terminal sequence of this sulfite oxidase core is reported (34 residues). It shows a strong similarity to that of liver microsomal cytochrome b5 and bakers' yeast cytochrome b2 core. The sequence comparison is discussed in terms of structural similarity to cytochrome b5. Our data suggest a common evolutionary origin for the three b-type cytochromes.