SUMMARY The binding characteristics of transcortin have been measured in 16 normal individuals, 10 pregnant women, five subjects taking oral contraceptives or on oestrogen therapy and in six patients with advanced liver disease. The affinity of transcortin for cortisol as measured by its equilibrium constant was found not to differ significantly between the normal individuals and the rest. The transcortin capacity was greatly increased in pregnancy and by oestrogen therapy but was normal in the cirrhotics studied. Despite the increase in total plasma cortisol in pregnancy, the unbound plasma cortisol concentration was only slightly raised and no increase was noted in the oestrogen-treated group. One of the 'normal' group was found to be transcortin deficient with low total plasma cortisol but the concentration of unbound cortisol and the rate of cortisol secretion were normal.