To determine the pathogenesis of interstitial nephtitis, immunopathological studies were performed with kidney and serum from a six-year-old boy. The kidney revealed linear staining of tubular basement membranes with antiserums specific for human IgG and C3; the membranes also showed fixation of heterologous complement. After incubation of the patient's serum on frozen sections from normal human kidneys, linear staining with IgG was detected by indirect fluorescence. This staining was eliminated by absorption of the serum with purified tubular basement membranes, but was unaltered by absorption with purified glomerular basement membrane. The antibody reacted with mouse, rabbit, dog, sheep, monkey, Sprague-Dawley and Lewis/Brown Norway rat tubular basement membranes, but not with that of Lewis rat or guinea pig, or with human skin, thyroid, parotid, lung, liver or pancreas. Anit-tubular-basement-membrane antibody may be involved in the pathogenesis of the patient's disease.