The enzyme horseradish peroxidase is routinely used in immunohistochemistry to facilitate the chromogenic oxidation of 3,3'-diaminobenzidine, producing an insoluble brown precipitate marking the location and quantity of a tissue protein. In an effort to develop non-protein reagents for tissue diagnostics, we have developed a peroxidase DNAzyme construct that can be used as a chromogenic functional group in immunohistochemistry assays. The DNAzyme is based on a reported 18-mer G-quadruplex-forming DNA sequence, PS2.M, and has been covalently linked to its catalytically active moiety, hemin, to avoid the high background signal associated with the use of free hemin in histochemical studies. The activity of the covalent construct is maintained under conditions where G-quadruplex formation is unfavored and where the noncovalent DNAzyme-hemin complex has no activity.