The enzymes of the citric acid cycle show at least two levels of organization within the mitochondrial matrix. Six of the possible eight sequential enzymes show specific interactions in vitro. Further, the enzymes bind specifically to the matrix surface of the inner membrane. A slightly damaged mitochondrial particle has been isolated which contains bound, but exposed, Krebs citric acid cycle enzymes. This particle (a metabolon) shows a kinetic advantage for two coupled systems, fumarate oxidation and malate conversion to citrate, over a solubilized system. N.m.r. experiments indicate that many components of the matrix are in a bound state.