In most of the children with short stature, no organic basis can be found, which has led to the suggestion that some cases of growth failure may be due to an immunoreactive but bioinactive growth hormone (GH). In order to compare GH immunoreactivity and bioactivity (measured as receptor binding ability), a radioreceptor assay allowing the measurement of GH levels in human serum was developed using cultured human lymphocytes and <sup>125</sup>I-labeled human GH purified by high-performance liquid chromatography. Serum samples were obtained after pharmacological stimulation with either insulin, glucagon or GRF from 19 healthy control subjects and 114 patients with various growth disturbances, aged 2.3–24.8 years. In general, there was a good correlation between the GH levels measured by the two methods, the RRA-GH levels being lower than the RIA-GH levels at all times irrespective of the stimulation test. In all the groups studied, most of the individual RRA/RIA ratios were within normal limits. It is concluded that the presence of an abnormal (bioinactive) GH molecule is extremely rare in patients with short stature.