ABSTRACT In a series of 50 breast tumours, oestrogen receptor (ER) measurements obtained by a single saturating dose (SSD) dextran coated charcoal assay were compared to those obtained by Scatchard analysis. In 45 ER+ve tumours there was good correlation between the two methods thus validating the SSD for ER measurement. However, Scatchard analysis revealed considerable heterogeneity in the affinities of the ER. This variation was most pronounced in tumours containing less than 60 fmol. ER/mg protein (Kd's ranged from 0.05 to >2nM). Analysis of tumours with higher ER contents showed a more homogeneous population with lower dissociation constants. Biphasic Scatchard plots were obtained in 10 out of the 50 tumours indicating the presence of at least two high affinity oestrogen binding moieties. It is suggested that variability in responses to endocrine therapy may be related to the heterogeneity of ER present in breast tumours.