Small biopsies of skin, and of keratinized and non-keratinized oral mucosa were incubated with horseradish peroxidase, fixed and reacted to demonstrate peroxidase activity at the light and electron microscope level. In all tissues peroxidase extended through the intercellular spaces of the epithelium up to, but not into, the stratum corneum of keratinized, or the superficial layer of non-keratinized, tissue. This distribution corresponds to that seen in studies using the tracer in vivo where the presence of an intercellular permeability barrier has been demonstrated in the upper cell layers. It is suggested that the method is a useful way of investigating the permeability of normal and pathological human epithelial tissues.