A highly sensitive method for light microscopic immunohistochemistry is described. The increased sensitivity compared with current methodologies is based on the horseradish peroxidase-catalysed deposition of biotinylated tyramine at the sites of immunoreactivity, followed by the detection of the biotin with streptavidin biotin horseradish peroxidase complex. This method is of general applicability in immunohistochemistry and has several important advantages over currently used immunohistochemical detection procedures. The most significant advantage is that several antibodies which to data have been non-reactive, even in antigen-retrieval formalin-fixed, was-embedded sections, now show strong and reproducible immunoreactivity using biotinylated tyramine amplification. In addition, many other antibodies can be used at significantly higher dilutions.