Human serum high-affinity growth-hormone-binding protein (GHBP), as determined by incubation with 125I-GH followed by chromatography on AcA 44 gel minicolumns, is lacking in patients with Laron-type dwarfism (LTD). We found that the specific binding of 125I-GH to high-affinity GHBP in normal human serum (m +/- SD) was 11.5 +/- 1.8% in 10 children 2-3 years old, 15.3 +/- 2.2% in 10 children 5-8 years old, and 19.3 +/- 2.9% in 15 adults 20-40 years old. It was 0.3% in a 2-year-old child with LTD, and 10.6 +/- 11.3% in his parents. It was 0.1% in another child with LTD, 7 years old, and 14.4 and 14.8% in his parents. The mean value in the heterozygous parents (12.8 +/- 2.1%) was significantly lower (p less than 0.001) than control values. A void volume peak (VVP) of radioactivity, corresponding to the so-called low-affinity GHBP which eluted at the void volume in chromatographs of normal sera remained unchanged with sera of patients with LTD or of their parents and appeared even after incubations of the tracer without serum. This study (1) shows that high-affinity GHBP is diminished in heterozygotes with LTD; (2) confirms that high-affinity GHBP and VVP are independently regulated, and (3) suggests that a part of the VVP may not be related to GH binding to some serum components.