To obtain a monoclonal antibody which is an antibody against a carrier proteinaceous polypeptide bonding to a somatomedin (a growth hormone) like polypeptide and useful for carrying out a specific assay in order to prevent activities of a carrier protein according to an immunoassay.

This monoclonal antibody is against a pure carrier proteinaceous prolypeptide other than a carrier protein subunit bindable to a somatomedinlike polypeptide (SM). The antibody is obtained by using a pure carrier protein subunit having SM binding properties as an antigen. The carrier protein subunit is prepared by successively subjecting a human blood plasma to an ion exchange chromatography, a hydrophobic interaction chromatography, a gel filtration chromatography, an affinity chromatography and a high-performance chromatography.