1. Portions of closed jejunal biopsies were gently homogenized in isotonic sucrose or sorbitol and centrifuged at 800 g for 10 min to prepare a cell extract.
2. The extract was fractionated in a single-step procedure by isopycnic centrifugation on a continuous sucrose or sorbitol density gradient with the Beaufay automatic zonal rotor.
3. The subcellular organelles were located in the density gradient by assay of marker enzymes and previously unassigned enzymes were localized to particular organelles.
4. The following organelles were characterized by their modal equilibrium densities in sucrose density gradients: brush borders (1·21), peroxisomes (1·18), mitochondria (1·16), endoplasmic reticulum (1·16), basal—lateral membranes (1·12). At least three distinct populations of lysosomes with different modal densities and enzyme content were demonstrated.
5. This analytical fraction technique can be used to study the subcellular pathology of human jejunum.