A radioimmunoassay was developed for somatomedin-binding protein isolated from human amniotic fluid. The mean level in amniotic samples from 20-22 week of gestation was high, 51 micrograms/ml. Human serum and serum fraction gave dose-response curves superimposable on that for the pure amniotic binding protein. Gel chromatography of serum at neutral pH disclosed immunoreactive binding protein only in fractions with a molecular size of 35 000 corresponding to elution volume for the low molecular form somatomedin-binding protein. The mean levels (mean and range) of immunoreactive somatomedin-binding protein in cord blood (191 ng/ml, 55-1698 ng/ml) and in diabetic patients with uraemia (97 and 51-174 ng/ml) were 5- to 10-fold elevated above those of healthy adults (23 and 18-36 ng/ml). In acromegaly the levels decreased with increasing GH production (r = -0.77; P less than 0.005). In adult patients with GH deficiency the levels were 2-fold elevated in comparison with healthy subjects. Apart from patients with uraemia a negative correlation was found in adults between the levels of immunoreactive binding protein and immunoreactive IGF-I which reflect the GH production (r = -0.81, P less than 0.001).