The acid-stable subunit of the GH-dependent large mol wt somatomedin-binding protein (SmBP) was isolated from human plasma Cohn fraction IV by a three-step procedure, and a specific RIA was developed which allowed measurement in unextracted serum. Although in normal human serum most of immunoreactive material was present as the large mol wt complex (150K), considerable amounts of smaller components were found by high performance liquid exclusion chromatography in the 60K, 42K, and 32K range. Normal serum levels were low at birth, rose sharply during the first weeks of life, and showed a moderate peak at puberty. To assess the diagnostic efficacy of SmBP for GH deficiency (GHD), patients previously diagnosed as GH-deficient by conventional criteria (n = 132) were compared to short statured children without GHD (n = 130). Taking the fifth centile as a limit of normality the majority of patients with GHD had subnormal levels, yielding high sensitivity of the test (0.97). In contrast, most of the non-GH-deficient children had SmBP levels within the normal range, resulting in high specificity (0.95) and, consequently, high accuracy (0.96). These results suggest that the large mol wt SmBP is an excellent screening parameter and is highly informative for GHD.