The binding of oestradiol to a nuclear fraction extracted from human breast carcinomatous tissue was demonstrated. The material, which was extracted with KCl, sedimented at 3--4S and bound oestradiol with high affinity (dissociation constant approximately 2 X 10(-10) mol/l). Oestriol, diethylstilboestrol and 5 alpha-dihydrotestosterone (100-fold excesses) competed with [3H]oestradiol for the binding sites (binding inhibited by 89 +/- 8 (S.D.), 92 +/- 6 and 57 +/- 8% respectively), whereas progesterone and cortisol (100-fold excesses) did not (binding suppressed by 5 +/- 5 and 2 +/- 3% respectively). Similar competition patterns were found for cytoplasmic material which bound oestradiol. The binding occurred at 4 degrees C and was therefore considered to be a measure of the amount of binding material unoccupied by endogenous oestrogen, Unoccupied binding sites for oestradiol in the nucleus and cytoplasm were measured in 35 samples of breast carcinomatous tissue using sucrose gradient centrifugation. In 17 out of 35 tumorus, unoccupied nuclear and cytoplasmic 8S and 4S binding sites could be detected. Three out of 35 tumours contained unoccupied nuclear binding sites and 4S cytoplasmic binding sites. Nuclear binding sites only were found in two out of 35 tumours. Unoccupied nuclear binding sites were not detected in 13 out of 35 tumours and ten of these tumours also did not contain unoccupied cytoplasmic binding sites.