Cytochrome with an alpha-band absorption maximum at 556 nm (77K) in the reduced minus oxidized spectrum is observed in membranes from Escherichia coli grown aerobically on most carbon sources. Previous work has suggested that this adsorption peak is due to cytochrome b556 of succinate dehydrogenase and to cytochrome omicron. We show here, by partial purification of the membrane cytochromes, that at least two other cytochromes with absorption maxima at 556 nm contribute to this peak. One of these cytochromes is associated with growth on DL-lactate. The other is formed under conditions of low aeration and has hydroperoxidase activity.