A radioimmunoassay (RIA) for human corticosteroid binding globulin (CBG) has been developed using 125I-labelled CBG and a monospecific solid-phase CBG-antiserum (CBG-Ab-cellulose). In an RIA of serum CBG concentrations, pure CBG standards (1-100 ng protein) or samples (1:200) were incubated (16 h at 20 degrees C) with 125I-labelled CBG and CBG-Ab-cellulose. After addition of 2 ml 0.9% NaCl, the tubes were centrifuged, supernatants were aspirated and the 125I-labelled CBG bound to the CBG-Ab-cellulose pellet was counted. The specificity of the RIA was confirmed by parallel displacement curves for serial dilutions of male, female and pregnancy sera, as well as pure CBG standards. The mean +/- S.D. recovery (99 +/- 8%) of pure CBG (1.6-25.0 ng) added to a diluted serum sample verified the accuracy of the method, and a good correlation (r = 0.97; n = 43) existed between serum CBG cortisol binding capacity (nmol/l) measurements and CBG concentrations (mg protein/l) measured by RIA. Intra- and interassay precisions (C.V.) at low to high serum CBG concentrations were less than 5% and less than 9% respectively. The mean +/- S.D. serum CBG concentrations (mg protein/l) measured by the RIA were: 21.8 +/- 4.6 in boys (n = 12), 20.0 +/- 4.2 in girls (n = 9), 20.7 +/- 2.7 in men (n = 6), 20.5 +/- 2.9 in women (n = 6) and 47.1 +/- 10.5 in pregnant women (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)