Holocarboxylase synthetase was purified 18,000-fold from bovine liver cytosol by a sequence of ammonium sulfate fractionation, alumina C gamma fractionation, DEAE-Sepharose CL-6B, EAH-Sepharose 4B, Sephacryl S-200 HR, Bio-Gel HTP, and Phenyl-Superose HR 5/5 chromatographies. Holocarboxylase synthetase activity was assayed using apopropionyl-CoA carboxylase from a patient with holocarboxylase synthetase deficiency as a substrate. Apopropionyl-CoA carboxylase was easily prepared from cultured lymphoblasts from this patient. Enzyme activity coincided with a 64,000-Da protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Additionally, the molecular mass of the native enzyme was estimated to be 60,000 Da by gel filtration on Sephacryl S-200 HR. These results suggest that purified holocarboxylase synthetase from bovine liver cytosol is a monomeric enzyme. Its Km value for biotin was estimated to be 13 nM.