Respiration in methanogenic bacteria appears to represent a metabolic system that is closely associated with electron transport phosphorylation. Cell extract was fractionated into two protein components, an oxygen-sensitive protein with hydrogenase activity and an oxygen-stable protein, as well as a heat-stable, dialyzable, oxygen-sensitive factor. The fluorescent compound could be used as an indicator for the safe storage of cells. In the presence of hydrogen, a variety of electron acceptors could be reduced by crude extracts. When extracts were passed through a column of diethyl aminoethyl-cellulose, they were resolved for a component that was required for reduction of nicotinamide-adenine dinucleotide phosphate (NADP). NADP was the preferred electron acceptor for glyceraldehyde-3-phosphate dehydrogenase, but malate dehydrogenase was most active with nicotinamide adenine dinucleotide. Addition of uncouplers of electron transport phosphorylation to the cell suspension caused a decrease in pool size. P. Cheeseman was the first to observe that cells of a methanogen.