To construct an immunochemical assay of human growth hormone (hGH) is difficult due to the complicated nature of the hormone. Several forms coexist in the circulation with a 22,000 molecular weight form dominating. Related hormones with up to 93% homology are also present. hGH with its 191 amino acids presents more than 10 epitopes. Furthermore, the existence of two binding proteins adds to the complexity of the assay. Despite intensive efforts involving establishment of international standards, analytical consensus has not yet been reached. It is concluded that more specific immunoassays should be established.