A staining procedure for glucose oxidase (GOD) as marker in light and electron microscopy is described. The procedure involves a coupled enzyme technique in which H₂O₂ generated by the marker enzyme (GOD) is serving as substrate for horseradish peroxidase (HRP). In the final reaction sequence, HRP utilizes H₂O₂ for the oxidation of diaminobenzidine (chromogen). The cytochemical reaction can be applied to all oxydoreductases which upon action on their substrates will generate H₂O₂ such as galactose oxidase and L-aminoacid oxidase.