The present study performed on a total of 567 cases of human female breast cancer compares the results of the biochemical assay (dextran-coated charcoal assay = DCC) for oestrogen receptor (ER) with those of several morphological methods developed for the detection of the ER or for the prediction of prognosis by use of other systems (FSA = fluorescent ligand binding assay, ER-ICA = monoclonal antibody assay for ER, LRA = lectin receptor assay using peanut agglutinin, and Barr body estimation). Whereas no correlation at all was observed among the results of the DCC and those of the FSA and Barr body estimation, the ER-ICA and the LRA showed an unanimous tendency towards higher values of ER with increasing intensity of the staining product. The results of the ER-ICA may be expressed by an immuno-reactive score (IRS) calculated from the staining intensity (SI) and the percentage of positive cells (PP). The morphological methods are evaluated with special regard to their correlation with the DCC, their theoretical basis, and their practical application. In summary, the ER-ICA appears to be the sole method directly visualizing the ER protein and--in contrast to the DCC--is therefore completely independent of the content of endogenous or exogenous oestrogens in the tumor tissue. The LRA provides valuable additional information concerning tumour differentiation and possible response to endocrine therapy, whereas the FSA and Barr body estimation should be considered as obsolete and should therefore be abandoned.