UNTIL recently it was generally assumed that so called 'resting' cells, suspended in a buffer solution in the presence of a nitrogen-free substrate, could be employed in studying respiration separately from other metabolic processes. Some doubt as to the validity of this assumption is suggested by the studies of Cook and Stephenson1, which show that the oxygen consumed during the oxidation of glucose, lactate or acetate by Bact. coli amounts to only from two thirds to three fourths of that required for complete oxidation.