Additional information and some modifications of an earlier published method ((1972) Z. Klin. Chem. Klin. Biochem., 10, 502-508) for the assay of oestrogen receptor in human breast tumours are presented. The correlation between the results obtained with this method and with the electrophoresis procedure of Wagner ((1972) Hopper Seyler's Z. Physiol. Chem., 353, 1235-1245) was found to be good. The use of glycerol stabilized calf uterus extract for quality control is proposed. Various factors affecting the assay and the discrimination between receptor positive and receptor negative are critically discussed.