The present study was undertaken to investigate the possible regulatory effect of chronic exposure to human growth hormone (hGH), in patients with acromegaly, on growth-hormone-binding protein (GH-BP). Nineteen patients with active acromegaly, before, during or after treatment, comprised the subjects of this study. Serum GH was measured by radioimmunoassay and GH-BP by a binding assay with dextran-coated charcoal separation. The specific binding of [125I]hGH (1 ng) obtained with 50 microliters serum was expressed as a percentage of total cpm. To evaluate the impact of the lower GH-BP on GH activity, we studied the effect of acromegalic serum on hGH displacement of [125I]hGH binding to GH receptors in rabbit liver membranes. Compared to normal controls (11.43 +/- 0.37%), the acromegalic patients had low serum levels of GH-BP (5.45 +/- 0.40%; p < 0.001), which correlated negatively with serum GH levels (p < 0.01). In 7 patients, GH-BP normalized within 2-3 months of successful therapy. The lower GH-BP was due to a reduction in binding capacity, whereas binding affinity remained unchanged. Acromegalic serum, with its low GH-BP, resulted in a shift to the left of the GH displacement curve when compared with normal human sera: IC50 values were 7.47 +/- 0.29 and 11.19 +/- 0.84 ng (p < 0.02) for acromegalic and normal human sera, respectively. We conclude that acromegaly is characterized by low levels of GH-BP due to a decrease in serum-binding capacity. The decrease in GH-BP may render the acromegalic serum GH relatively more active in the GH receptor assay.