In many immunohistochemical studies horseradish peroxidase (HRP) is used as the marker enzyme. An in situ and indirect quantitation of antigen or antibody would be possible by determination of the HRP activity. For this reason quantitative aspects of histochemical reaction procedures to localize HRP activity have been studied in a model system consisting of polyacrylamide films into which the enzyme had been incorporated. A linear relation between the amount of HRP activity and the spectrophotometrically determined amount of reaction product could be established for reactions using 3,3'-amino-9-ethylcarbazole as the hydrogen donor. A similar relationship could not be found after staining with benzidine-containing media. These results indicate that for certain substrates the quantities of dyes formed during the cytochemical HRP reaction can be considered as a measure for the activity of the enzyme. This opens perspectives for the in situ quantitation of antigens or antibodies.