This study aimed at assessing the usefulness of a new monoclonal antibody (1D5) for the demonstration of oestrogen receptors (ER) in paraffin wax sections, using brief microwave processing rather than proteolytic predigestion. Routinely processed paraffin wax sections of 50 cases of breast carcinoma with known ER concentrations, estimated by the standard dextran-coated charcoal (DCC) biochemical assay, were examined using the avidin-biotin complex-immunoperoxidase technique. The results were assessed semiquantitatively, using a five grade scoring system. Of the 50 cases examined, 37 were positive and six were negative by both DCC and immunohistology. Of the remaining seven cases, three (6%) were negative by DCC but positive with immunohistology, and four (8%) were positive with DCC and negative with immunohistology. The DCC results of the latter four cases were 10, 14, 14 and 16 fmol/mg protein which is at the lowest level of positivity, our cutoff point being less than 10 fmol. The monoclonal antibody 1D5, as used in this study, can provide easily assessed reliable information about the ER status of breast carcinoma using routinely processed paraffin wax sections.