Tight junctions have not been described in the adult human glomerulus. In a 22-year-old patient with kappa light chain proteinuria, tight junctions were observed between glomerular epithelial cell foot processes (kidney biopsy). The light chain isolated from urine proved to have a relatively high isoelectric point. When the light chain was exposed in vitro to the mucosal surface of a ‘leaky’ epithelium, the Necturus gallbladder, transepithelial resistance and potential difference increased in a concentration-dependent, reversible manner and NaCl dilution potentials decreased, consistent with a reduction in tight-junctional ionic permeability. Gallbladders fixed in situ and freeze-fractured during peak electrophysiological responses revealed an increase in tight-junctional depth. This report indicates that certain light chains, possibly by virtue of a positive charge, may induce changes in epithelial tight junction structure and/or permeabil