Nephritis in rats was induced by intravenous injection of purified ferritin-conjugated rabbit and duck nephrotoxic globulin. Using the fluorescent antibody technique, the same capillary pattern was found as that in glomeruli of rats receiving uncoupled nephrotoxic globulin. Electron microscopy revealed a heavy accumulation of the basement membrane-fixed antibody almost exclusively at the endothelial side. A higher concentration of ferritin was demonstrable in the peripheral basement membrane. The once-fixed antibody remained at the site of reaction though decreasing with time. The half-disappearance time seemed to be shorter than that of the uncoupled nephrotoxic globulin. No difference in localization was observed between rabbit and duck antibody. At least 40 basement membrane-fixed antibody molecules from the rabbit per 3000 mµ 2 of filtration surface were needed to cause immediate nephritis. To induce nephritis using duck antibody, a greater number of basement membrane-fixed antibody seemed to be necessary. No evidence of specific reaction with constituents of glomerular cells was obtained.