In our previous description of the circulating GH-binding protein (GH-BP) we observed, in addition to the main GH-BP complex, a second macromolecular component designated peak I during gel filtration of [125I]GH-plasma mixtures. This component was not further characterized because of its small magnitude, seeming nonsaturability, and suspected artifactual nature. We have now characterized peak I as the complex of a low affinity BP with GH. To gain a better knowledge of the nature of peak I, whole plasma or plasma fractions containing isolated peak I-BP (ammonium sulfate precipitated or prepared by gel filtration of plasma) were incubated with monomeric [125I]GH and varying concentrations of unlabeled GH. The mixtures were then analyzed by Sephadex G-100 chromatography to separate free from protein-bound GH. Peak I-associated radioactivity was saturable at high concentrations of human GH, but not with animal GHs. Saturation/Scatchard analysis yielded an association constant of 10(5) M-1 and a maximum binding capacity of 15 mg/L plasma. Chemically cross-linked complexes of [125I]GH with isolated peak I BP were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, isoelectric focusing, and two-dimensional electrophoresis. These experiments yielded a mol wt of 124 kD and a pI of 7 for the cross-linked complex. This is in contradistinction to the previously reported high affinity GH-BP complex, which when cross-linked has a mol wt of 76 kD and a pI of 5.(ABSTRACT TRUNCATED AT 250 WORDS)