By using biotin labelled proteoglycan core protein and an avidin enzyme system, hyaluronan (hyaluronic acid) was visualised in specimens of human jejunum. Intense staining for hyaluronan was seen in the loose connective tissue of the villi and of lamina propria while the epithelial layer was unstained. The muscularis mucosae showed only faint staining. The accumulation of hyaluronan in the subepithelial layer of the jejunal mucosa indicates that the previously reported high jejunal secretion of hyaluronan is due to passive diffusion from the subepithelial interstitium. The physicochemical characteristics conferred by hyaluronan may be important for the villi function.