An easy, rapid and precise technique for the determination of the binding capacity of the sex hormonebinding globulin (SHBG) is described. The plasma is incubated with [3H]5 alpha-dihydrotestosterone (DHT) either alone, or together with an excess of unlabelled DHT. The SHBG-binding peaks are demonstrated by agar gel electrophoresis according to Wagner (1972) Hoppe-Seylers Z. Physiol. Chem. 353, 1235--1245), and the SHBG-binding capacity is calculated from the radioactivity specifically bound to the SHBG peak, corrected for the dissociation of the [3H]DHT from SHBG during the 90 minute electrophoresis. In addition to investigating various problems of methodology, the following clinical results were obtained: 1 Normal range (x +/- s) of the SHBG-binding capacity for males (age 22--41; n = 26) 38 +/- 11 nmol/1.2. An older male group has, with 48 +/- 10 nmol/l, a a significantly higher binding capacity. 3. The highest values found so far were in pregnancy plasma of the third trimenon (330 and 400 nmol/l).