Biopsies or mastectomy specimens from 69 malignant and 17 non-malignant human breast tumours have been examined with respect to cytoplasmic oestradiol receptors and endogenous oestradiol concentration. Of the malignant tumours, 45 (65%) had significant oestradiol receptor concentrations (136.7 fmol/mg protein +/- 38.3 S.E.M.). Oestradiol values in the cytosol were not correlated to receptor levels. Cytosol oestradiol concentrations in the receptor-negative tumours were normally distributed about a mean value of 7.4 fmol/mg protein +/- 0.9 (S.E.M.). Oestradiol levels were similar to the median concentration of receptor found in tumours from younger women, and consequently may influence receptor measurements in such tumours. In receptor-positive cytosols a much wider range (less than 1 to 69.7 fmol/mg protein) and a non-symmetrical distribution of oestradiol values were found which closely corresponded to the range of occupied receptor concentrations previously measured in tumour cytosol. When tumours having undetectable oestradiol values were excluded, receptor-positive cytosols had significantly higher oestradiol concentrations than those found in receptor-negative tumours (P less than 0.01). No significant difference in cytosol oestradiol concentration was found in receptor-positive and negative pre- and postmenopausal women. This would indicate that factors other than plasma levels influence tissue availability of oestradiol.