A particulate fraction, showing catalytic activity towards intermediates of the Krebs cycle, has been isolated from pea seedlings by the method of differential centrifugation. The particulate material, possibly consisting of mitochondria, appears homogeneous under the microscope and is stained by Janus Green B. The enzyme system necessary for the integrated and single step reactions of the Krebs tricarboxylic acid cycle is described, and the requirenients for cofactors, discussed.