Human duodenal epithelium, at the level of the crypt. Fixation was performed in buffered 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 black granules located at the apical side of the elongated epithelial nuclei. A few rounded goblet cells are recognized. The brush border is faintly seen. In the subepithelial capillaries, 4 large structures are erythrocytes.