Amino acid sequences of 6 mitochondrial cytochromes b are analysed to locate the binding sites of the two protohaems. Four invariant histidines are found in two protein segments which span the membrane. In each, two histidines are separated by 13 residues. This would place them on the same sides of alpha-helices, and the protohaems could be sandwiched as bis-imidazole complexes between the two transmembrane segments. In this model the haems are located in different halves of the bilayer; the Fe-Fe distance is about 20 A.