The cytochromes of the filamentous bacteria Streptomyces clavuligerus and Saccharopolyspora erythraea have been studied by low-temperature (77K) difference spectra and room-temperature potentiometric titrations. Difference spectra of membranes from Sa. erythraea indicate the presence of two c- and two b-type cytochromes. A further b-type cytochrome is revealed by potentiometric titration. The soluble and membrane-bound forms of a green pigment, believed to be involved in electron transport to oxygen, are each shown to have complex absorption spectra which, in the case of the soluble form, may be potentiometrically resolved into several components. Three b-type and three c-type cytochromes are shown to be present in the membranes of St. clavuligerus.