A monoclonal antibody-based immunoenzymometric assay (IEMA) for the measurement of human serum growth hormone is described. Two high-affinity and complementary monoclonal antibodies were selected from a panel of 9 obtained upon fusion of SP2/O myeloma cells with spleen cells from a Balb/c mouse immunized against human growth hormone of pituitary origin. One monoclonal antibody was immobilized by attaching it to the walls of microtiter wells and the second was biotinylated. The reaction was quantitated by the addition of streptavidin-peroxidase. The sensitivity of the assay was 0.2 mIU/l and the intra- and interassay coefficients of variation for 4.6 to 46 mIU/l were less than 8.3 and 17.3%, respectively. Cross-reaction with human placental lactogen, human prolactin and rat growth hormone was less than 0.1% (w/w). Comparison of results obtained for 180 routine serum assays by radioimmunoassay and the assay described here had a correlation coefficient of 0.94 with a mean value of 16.3 +/- 1.3 (mean +/- SEM) and 13.3 +/- 1.2 mIU/l, with the IEMA providing values 18% lower than the RIA. The discrepancy emphasizes the necessity of redefining normal ranges before immunometric assays, like the one described, can be used routinely.