The recent development of an immunofunctional assay for GH-binding protein (GHBP) facilitates measurement of GHBP in biological fluids. Previous methods employed GH binding followed by size exclusion chromatography to determine GHBP levels (GH binding assay), and a considerable body of information exists based on data obtained with that type of assay. To cross-validate the two methods and permit comparison of results obtained in the two assays, we measured GHBP by both methods in 61 plasma samples from normal adults (aged 19-69), 10 patients with acromegaly, 2 patients with Laron dwarfism, and in a normal adult plasma pool. The results show a good overall correlation between the two methods (r = 0.669). However, for individual observations, the coefficient of determination was not high enough to permit interconversion of data with high precision. There is both biological and methodological variation in GHBP levels, rendering the interpretation of a single observation difficult except in the extreme range. The range of values was wider in the immunoassay (56-1187 pmol/L) than in the GH-binding assay (12.1-36.1% GH bound/400 microL). There was no significant sex difference in plasma GHBP levels, nor was there an age-dependent trend in adult subjects as assessed by both assays. Patients with acromegaly had significantly decreased GHBP levels in both assays, but the majority of the values were within the low normal range. The two patients with Laron dwarfism had undetectable GHBP in both assays. The previously defined unit of GHBP (i.e. the amount contained in 1 ml pooled adult plasma) corresponds to 256 fmol GHBP as determined by immunofunctional assay.