To separate mannan-binding lectin (MBL) and MBL-associated serine proteases (MASPs) from human plasma.A two-step affinity chromatography on underivatized sepharose 4B was employed for purification of MBL-MASP complex, followed by gel filtration on a Sephacryl S-300 column for separation of MBL and MASPs from the complex. The purification procedures were performed at 4 degrees Celsius with the addition of two proteolytic inhibitors, phenyl methylsulfonyl fluoride and 1,10-phenanthroline during affinity chromatography but not in the gel filtration buffer.Preparations of highly purified MBL and proenzyme MASPs were obtained. The purified MBL was shown by SDS-PAGE and Western blotting to be a functional multimer composed of 28,000 and 32,000 peptide chains, with high bioactivity as demonstrated by ligand-binding assay and yeast agglutination experiment.A simple and convenient procedure is established successfully for the purification of MBL and the proenzyme MASPs.