Hepatic citrate synthase activity has been shown to be increased 2- to 3-fold in vitamin B12 deficiency. Immunochemical titrations of the affinity chromatography-purified enzyme obtained from liver of animals with B12 deprivation demonstrated that this increase in activity was the result of a true increase in enzyme protein content. When fixed ratios of aliquots of normal and B12-deprived rat liver homogenates were mixed, the activity measured showed no change from the expected total citrate synthase activity based on the admixture ratios. Partial purification of the enzyme resulted in the expected recovery of the enzyme at each of the purification steps. Thus, it is unlikely that the change in enzyme activity in B12 deprivation was due to the presence of a soluble or easily dissociable normally occurring activator or inhibitor. Ouchterlony double diffusion studies, immunochemical titration, and determination of Km vlaues for exalacetate and acetyl-CoA (substrates for citrate synthase) and Ki values for ATP (inhibitor of citrate synthase) all indicated that the enzyme from the B12-deprived livers was structurally the same as that from normal liver. Hepatic citrate synthase degradation rate constants were shown to be essentially unchanged in B12deficiency. The rate of hepatic citrate synthase synthesis, under steady state conditions, was shown to be 2.8-fold greater in the B12-deficient animal than in the normal animal. The increased rate of synthesis appeared to explian the increased enzyme content. Finally, no change in specific activity of the enzyme was seen in brain, heart, or kidney in the B12-deprived animal.