In rat mammary tumours and uteri it was found that reaction time, ionic strength, reproductive state and storage in liquid nitrogen affected the form of the oestrogen receptor detected by sucrose density-gradient analysis using a vertical-tube rotor. A standard method of gradient analysis was defined. The sedimentation profile of the oestrogen receptor was compared in three types of experimental rat mammary tumour of known hormonal sensitivity. These were two lines of ovary-independent transplantable tumours and those tumours induced by dimethylbenz(a)anthracene (DMBA) which are mainly ovary-dependent. In all three types of tumour the 8S receptor was the predominant molecular form (32 out of 36 tumours induced by DMBA and 11 out of 13 transplantable tumours). Sedimentation profiles of the oestrogen receptor were also examined in 46 human breast carcinomata found to be receptor-positive by the dextran-coated charcoal technique. Of these, 17 were found to be receptor-negative by gradient analysis and 28 out of 29 receptor-positive tumours contained 8S receptor. It was concluded that (1) 8S receptor is the predominant molecular form in both human and rat mammary tumours, (2) molecular form of receptor is not related to ovarian dependence in these rat mammary tumours and (3) gradient analysis seems unlikely to provide additional discrimination in the prediction of response to endocrine therapy over that provided by simpler methods.

Forty-eight rat mammary tumours and 25 human breast carcinomata were examined for (a) oestrogen receptor activity and (b) capacity for intranuclear translocation of oestrogen. Receptor activity was determined by saturation analysis using charcoal adsorption to separate free and bound hormone. The capacity for intranuclear translocation was determined by incubation of tumour slices in Krebs–Ringer bicarbonate solution containing [³H]oestradiol-17β at a physiological concentration, in the presence and absence of a large excess of non-radioactive oestradiol, at 37 °C. Saturable nuclear uptake of [³H]oestradiol (= translocation) in boiled or receptor-negative tissues was minimal, i.e. <12 fmol/mg DNA per 2 h and in receptor-positive tissue was reduced by 85% when the temperature was lowered to 0°C. In 27 ovary-independent transplantable tumours and 21 dimethylbenz(a)anthracene-induced tumours (predominantly ovary dependent) saturable nuclear uptake was strongly correlated with level of oestrogen receptor activity (Spearman's rank correlation coefficient r= +0·73, P<0·001). Nineteen of these 48 tumours were examined further: 60–80% of the saturable uptake was precipitable with protamine sulphate and this fraction of the total saturable uptake was also highly correlated with receptor level (Spearman's r = +0·87, P<0·001). In the 25 human tumours studied, saturable nuclear uptake was again correlated With receptor level (Spearman's r = +0·75, P<0·001). These studies suggest that saturable transfer of oestradiol from the extracellular medium through the cytoplasm to the cell nucleus is mediated by the oestrogen receptor in the rat and human tumours examined. They provide no evidence for any defect of the receptor mechanism before nuclear binding in tumours which are receptor positive but hormone insensitive.