A selection scheme based on the nutritional requirements of a previously described respiration-deficient Chinese hamster line has been used to isolate new mutants defective in oxidative energy metabolism. Three of the primary characteristics of this type of mutant are (1) a strict dependency on the continued presence of glucose for survival; (2) a drastic reduction in the rate of oxygen consumption; (3) an inhibition of Krebs cycle activity resulting in auxotrophy for asparagine and carbon dioxide. In the case of one cell line which was used (V79), up to 65% of the survivors of a selection were found to possess this phenotype after only one round of selection. By contrast, it proved much more difficult to obtain such mutants from another cell line (CCL16). A preliminary characterization of a number of these mutants is presented.