The antigenic surface of the human growth hormone (hGH) molecule has been characterized by means of monoclonal antibodies (MAbs) and their specific binding sites. From a panel of MAbs raised against the 22 kDa variant of hGH, ten distinct antigenic entities could be identified as binding sites for specific MAbs in an epitope mapping experiment. The experiment was based on combinations of the MAbs in two‐site assays. By investigating the cross‐reactivity of the MAbs with growth hormone (GH) variants and fragments of hGH, the underlying protein structure of some of the partially overlapping epitopes could be identified. By using antibodies of known epitope specificity as competitors in receptor binding assays of hGH, it was demonstrated that the lactogenic and somatogenic biological effects of hGH are mediated through different portions of the hormone. These observations may explain some of the differences between the various immunoassays for hGH. A strategy for selecting antibodies of known epitope specificity and biological relevance for the design of plasma GH immunoassays is outlined. The results of such an immunoassay should closely reflect the biological activity of circulating plasma hGH.