The demonstration and partial characterization of a high-affinity saturable binding component for oestradiol-17 beta in the cytosol and nuclear extract of the benign hypertrophied human prostate is reported. This binding component was found to precipitate with protamine sulphate and to exhibit marked specificity for oestradiol and diethylstilboestrol (DES) but not dihydrotestosterone (DHT) or progesterone. Analysis of oestradiol-labelled cytosol on low ionic strength glycerol gradients revealed a binding component with a sedimentation coefficient of 4S which was inhibited with DES but not DHT and destroyed either by preheating labelled cytosol at 45 degrees C for 30 min or by treatment with the sulphydryl blocking agent, parachloromercureobenzoate. The oestradiol-binding complex precipitated by ammonium sulphate was found to have an isoelectric point (pI) of 6.3 as analysed on polyacrylamide-gel plates. The oestradiol-binding component in the nuclear extract also exhibited similar steroid specificity and sedimented with a coefficient of 2.8S on high ionic strength glycerol gradients. Isoelectric focusing of the heparin-extracted nuclear oestradiol-binding complex revealed a pI of 6.0. The dissociation constants (Kd) and the concentrations of the cytoplasmic and nuclear oestradiol-binding sites in 19 samples of benign hypertrophied prostates were estimated by Scatchard plot analyses. The mean Kd and concentration of binding sites in the cytosol were 2.9 +/- 2.6 (S.D.) nmol/l and 11.7 +/- 8.4 fmol/mg protein respectively. The corresponding values for nuclear extract were 4.1 +/- 7.5 nmol/l and 347.1 +/- 415.5 fmol/mg DNA respectively. There was no significant difference between the mean concentration of oestradiol-binding sites in the two cellular fractions.