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Interactions of substrates, activators, and inhibitors are examined in the reaction catalyzed by the diphosphopyridine nucleotide specific isocitric dehydrogenase of the mitochondria of the Ehrlich ascites carcinoma. Cooperative homotropic interactions in rate-concentration functions are found for isocitrate, magnesium ion, and the effectors, adenosine diphosphate (ADP) and reduced diphosphopyridine nucleotide (DPNH). Isocitrate homotropic cooperativity is pH dependent; ADP abolishes this effect, while DPNH, a negative effcctor, increases isocitrate cooperativity. Reduced triphosphopyridine nucleotide inhibits the enzyme and manifests somewhat smaller cooperative interactions. ADP antagonizes the inhibition by both