Summary. Examination of oxidized and reduced spectra of isolated membranes (plasma and mesosome membranes) and protoplasmic fractions of the lysozymesensitive Cram-positive bacteria Bacillus Ucheniformk, Bacillus stearothcrmophilus, Micrococcus lysodeikticus and Sarcina lutea, has shown that virtually all of the cytochrome and in addition the carotenoids in the latter two organisms are localized in tbe membranes. Although the membranes isolated from cells of Micrococcus lysodeikticus and Sarcma lutea grown in the presence of diphenylamine still possessed the normal complement tif eytochromes bound to the membranes, the structures were devoid of the unsaturated carotenoids. "Walls" or "envelopes" of Escherichia coli contained a higher proportion of the eytochromes and NADH oxidase aetivity than that fount! in the protoplasm. NADH oxidase activity was also localized in isolated membrane structures of the other bacteria investigated.INTRODUCTION.