Current diagnostic standards for growth hormone deficiency were derived from measurement of growth hormone using polyclonal antibody assays. Recently, however, immunoassays using monoclonal antibodies to growth hormone have been commercially introduced. This study compares growth hormone levels obtained with two new monoclonal assays with levels obtained with a standard polyclonal assay. In 20 patients, the polyclonal assay (National Pituitary Agency) measured a mean growth hormone concentration of 14.0 +/- 10 (SD) ng/dl compared with monoclonal assay 1 (Hybritech) measurement of 10.4 +/- 7.2 ng/dl (P less than 0.01) and monoclonal 2 (Wellcome Co.) measurement of 18.9 +/- 9.8 ng/dl (P less than 0.01). The Hybritech monoclonal assay measured lower growth hormone levels than the polyclonal assay in all 20 patients. The Wellcome assay measured higher growth hormone levels than the polyclonal assay in 17 of 20 patients. There is not complete agreement between polyclonal and monoclonal antibody growth hormone assays in their measurement of plasma growth hormone levels. Clinicians using the new monoclonal antibody growth hormone assays will need to carefully develop separate diagnostic criteria for growth hormone deficiency that takes into account the disparity between the polyclonal assays that were used in the development of the current diagnostic standards and the new monoclonal antibody assays.