The ability of the bacteroids Rhizobium lupini isolated from the nodules at the stage of active nitrogen fixation to consume [1-14C]- and [2-14C] glucose was studied. 5 min after glucose addition to the cell suspension the label is rapidly incorporated into the organic acids, amino acids and sugars, which is indicative of glucose penetration inside the bacteroids and of a high rate of its catabolism. The rapid incorporation of the label into all Krebs cycle metabolites and the activities of citrate (si)-synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, which are positively correlated with the rate of the nitrogen fixation process in the course of plant vegetation provide evidence for the occurrence of the tricarboxylic acid cycle in the bacteroids.