Human (h) GH in plasma exists as a series of size isomers, which are in part explained by the presence of hGH oligomers. However, certain aspects of circulating large mol wt hGH, such as its high relative proportion compared to that in the pituitary, are poorly understood. To explore whether binding of hGH to plasma protein(s) could contribute to the phenomenon of large mol wt hGH, we incubated freshly prepared monomeric [125I]hGH or biosynthesized [3H]hGH with normal human plasma or serum at pH 7.4 for various time periods at 22 and 37 C. Plasma radioactive hGH patterns were then analyzed simultaneously with unincubated tracer hGH by Sephadex G-100 and G-200 chromatography. We found that part of the radioactivity was converted to a component with an apparent mol wt of 85,000, suggesting binding to a plasma protein(s). This phenomenon was inhibited in a dose-dependent fashion by unlabeled hGH. Saturation/Scatchard analysis indicated an association constant (Ka) of 2-3 X 10(8) M-1 and a maximum binding capacity of 20 ng hGH/ml plasma. Binding was rapid, reversible, and specific. A number of polypeptide hormones, including human placental lactogen, hPRL and rat GH, did not inhibit hGH binding. However, the 20K variant of hGH interacted weakly with the plasma binding component (Ka, 1.2 X 10(7) M-1; maximum binding capacity, 450 ng/ml). The binding component was heat labile and could be partially purified by gel permeation chromatography and affinity chromatography on a hGH-Sepharose column. Its estimated mol wt is 60,000-65,000, and it appears to bind one molecule of hGH to form a complex of 80,000-85,000 mol wt. The binding component is neither albumin nor an immunoglobulin. Under physiological conditions, a minimum of 15-18% of circulating hGH is presumably bound to this plasma component. We conclude that a specific, high affinity, low capacity binding protein for hGH with mol wt of 60,000-65,000 exists in normal and hypopituitary human plasma. hGH complexed with this protein forms part of big-big hGH. The biological significance of this binding protein remains to be investigated.