Human duodenal epithelium, at the level of the crypt. Fixation was carried out inbuffered formal-calcium. Peroxisomes are stained with DAB by their catalase activity at pH 10.5, postosmicated and embedded in epon. 2 µm plastic section.) Peroxisomes are the very small brown granules located between the brush border and the elongated epithelial nuclei (unstained). A few rounded goblet cells are recognized. In the subepithelial capillaries, 5 large black structures are erythrocytes that reacted with DAB by the peroxidatic activity of hemoglobin.