Several immunohistological methods for the demonstration of progesterone receptors were tried on routinely processed paraffin wax sections of breast carcinoma, using Abbott's PgR-ICA monoclonal antibody. The best results were obtained with the avidin-biotin-immunoperoxidase complex method with no prior trypsinisation or DNAse digestion, and with imidazole added to the final diaminobenzidine developing solution. A simple semiquantitative scoring system was used to assess the staining results which were then compared with the results obtained by a standard dextran-coated charcoal biochemical assay. Of 31 cases examined, the results of the two methods were concordant in 25 (81%) of cases. This is near the higher end of the concordance range obtained by several other authors using frozen sections. The discordance encountered in a few cases was possibly the result of sampling errors which are more likely to occur with the chemical rather than the histological method. It is concluded that the method described here is fairly reliable and would greatly simplify the process of assessment of progesterone receptors in breast, and possibly other tumours.