AbstractThe radioreceptor assay (RRA) of human growth hormone (hGH) was developed using liver microsomal fraction from estrogen‐treated female rats. The method allows measurement of physiological amounts of hGH (1.5–100 ng/ml) in unfractionated serum. Cross‐reactivity with human prolactin was 4.6%. Native hGH, recombinant methionyl‐hGH, and methionine‐free hGH showed a similiar binding to the receptors. Simultaneous determinations of serum hGH concentration with the RRA and radioimmunoassay (RIA) were performed in samples after the administration of GH‐releasing hormone (GRH) (1—44) from 15 normal short children and 25 patients with hGH deficiency. Overall RRA/RIA ratio in 146 samples was 0.84 ± 0.31 (SD). Of 15 normal short children, one showed a significantly reduced RRA/RIA ratio of 0.13 ± 0.03 in comparison with that of the other (0.70 ± 0.40) (p<0.001). His growth rate was accelerated by exogenous hGH administration, indicating that he secreted a biologically inactive hGH. The present RRA provides a useful method for screening bioinactive hGH in a large number of serum samples from children wi