Publisher Summary The chapter describes the method used routinely for purification of the enzyme from sheep liver. Several methods are available for the assay of methylmalonyl-CoA mutase. All operations are carried out at 0-4° unless otherwise stated. The protein values given in connection with the purification procedure are spectrophotometric values. The procedure gives a 5000-fold purification from the original extract with about 1% overall yield. Based on the refractometric determination of protein, 90 kg of sheep liver mince, prepared over a period of 8 months, yields 24 mg of concentrated enzyme of specific activity. This preparation is homogeneous in the ultracentrifuge. As the specific activity of the initial extract is of the order of 0.001, the overall purification is about 7000-fold. The purified holoenzyme preparation consists of 88% holoenzyme and 12% apoenzyme.