A number of methods for demonstration of peroxidase activity have been tested on immunocytochemical nitrocellulose models. By applying a well-characterized primary antibody and Sternberger's peroxidase-antiperoxidase technique, the sensitivity of various protocols has been evaluated. Best results were obtained with diaminobenzidine as chromogen, especially in conjunction with heavy metal salts, either added directly to the medium or used as "toners" of the end-product. Use of silver intensification of the diaminobenzidine/metal end-product increased sensitivity further.