Active particles, presumed to be mitochondria, have been isolated by differential centrifugation from the hypocotyls of etiolated soybean seedlings. These particles were capable of oxidizing all Krebs cycle intermediates tested, although fumarate and oxaloacetate were oxidized relatively slowly. Oxygen uptake in the presence of succinate, pyruvate, or ketoglutarate was increased two to four times by the addition of ATP, MgSO4, and cytochrome c. Maximum activity was obtained when both sucrose and phosphate were added to the grinding and suspending media, and when the reaction mixture was buffered near pH 7.0. Endogenous oxidation was reduced 50% by one washing and at the same time the oxidation of exogenous substrate was increased two to three times. Average P:O ratios of 0.4, 0.8, and 0.9 were obtained without added hexokinase or fluoride for succinate, pyruvate, and ketoglutarate, respectively. These ratios were increased to 0.8, 1.2, and 1.5 by the addition of hexokinase. In all cases, 10−2 M NaF decreased oxygen uptake 30 to 40% without affecting P: O ratios. Isolated soybean particles appear to be similar in most respects to particles from other plants. Certain differences were found and are discussed.