Experimental allergic interstitial nephritis was induced in Lewis rats, which were immunized with isologous kidney suspension.<BR>The immunization was performed by the intracutaneous and intraperitoneal injections of the 50% kidney suspension, emulsified in complete Freund's adjuvant (Perrin's modification) . Pertussis vaccine was also injected into the dorsum of feet and into the peritoneal cavity. Rats were nephrectomised or sacrificed at 7, 10, 14, 21, 28, 35, 42, 77, 84 and 91 days after the immunization.<BR>Clinical examination was performed by measurement of urinary protein, sugar and volume. Urinary volume and protein remained within normal range and no rats developed glucosuria through the period of the experiment. The susceptibility of the interstitial nephritis was found in the younger rats than the older.<BR>By light microscopical study, the infiltration of mononuclear cells and polymorphonu-clear leucocytes (PMNs) were found locally around the tubules (mainly proxymal convoluted tubules (PCT) ) at 10 days after the immunization and with time became diffuse throughout the renal cortex. The lesion almost abolished at the 35 th day.<BR>By direct immunofluorescent study, rat-IgG and C 3 was observed along glomerular capillary loops in granular fashion at the 35 th day.<BR>By electronmicroscopical study, infiltration of cells (predominantly lymphocytes) were found in the interstitial tissue and some of them attached to the basement membrane of the PCT at 10-14 th day after the immunization. Other abnomalities were those, such as focal wrinkling of tubular basement membrane (TBM), gaps formation between the TBM and the tubular cells, invasion of the lymphocytes inbetween the tubular cells and/or the tubular lumen, decrease or loss of infoldings of the PCT, presence of fat droplets in the tubular cells and swelling of the mitochondrias. The lesion seems to be abolished at the 28 th day, and almost disappeared except for fibrosing changes at 35 th day. On the other hand, at the 35 th day, glomerular basement membrane (GBM) showed irregular thickning which was composed of subepithelial dense deposits.<BR>These findings suggested that the interstitial nephritis was developed by mechanism of cellular immunity, e. g. the lymphocytes sensitized with certain renal antigen (s) may play a major role for tissue destruction and induction of infiltration of macrophages, PMNs and the other cells.