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-17 beta at a physiological concentration, in the presence and absence of a large excess of non-radioactive oestradiol, at 37 degrees C. Saturable nuclear uptake of [3H]oestradiol (= translocation) in boiled or receptor-negative tissues was minimal, i.e. less than 12 fmol/mg DNA per 2 h and in receptor-positive tissue was reduced by 85% when the temperature was lowered to 0 degree 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 less than 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 less than 0.001). In the 25 human tumours studied, saturable nuclear uptake was again correlated with receptor level (Spearman's r = +0.75, P less than 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.