With the procedure which uses gel-filtration and high pressure liquid chromatography (HPLC) the authors measured the GH-binding proteins in 96 healthy subjects aged 6 months to 40 years. In the blood 45% of the GH is bound to the BP. The diagnosis was confirmed by our method in 13 patients with Laron type dwarfism and in four patients the GH receptor defect had been proved. The parents, the brothers and sisters showed significantly lower GH-BP II (principal GH-BP) levels. Possible regulation of GH-binding proteins in human plasma was examined. Eight children with isolated GH deficiency had a very low level of plasma GH binding activity (mean +/- SEM) (10.2 +/- 1.1% of radioactivity). Under GH treatment the hormone binding to high affinity BP (GH-BP II) increased in every patient to reach the mean value of 18.5 +/- 1.4%. This increase was related to a higher binding capacity without any significant change in the binding affinity. In nine boys with pubertal delay, the GH specific binding to peak II-BP (GH-BP II) was found to be normal (30.6 +/- 3.7%); it decreased significantly following testosterone treatment. In four boys with precocious puberty, the specific GH binding to peak II BP was low (16.6 +/- 1.1%). It increased significantly to 21.6 +/- 1.1% of radioactivity after treatment with LH-RH analogue. GH and testosterone have an opposite role in the regulation of the high affinity GH-BP.