The invention relates to monoclonal antibody capable of specific binding to the molecular weight of 20 kDa variant of human growth hormone. This monoclonal antibody has no substantial binding to hGH of molecular weight 22 kDa. The invention also relates to the use of this monoclonal antibody for measurement of hGH 20K, especially in body fluids. The antibodies can be used for detection and quantification of hGH 20K, especially in serum.