Summary 1) A severe nephrotic syndrome was noted in 20 rats given Freund's adjuvants and a supernate of rat kidney suspension by intraperitoneal injection. When kidney tissue was replaced by liver, only 3 of 21 animals developed a much milder renal disease. Lung or muscle suspensions failed to induce proteinuria in 10 rats. 2) In large doses (0.5 ml) Freund's adjuvants alone produced a mild nephrotic disease in 3 of 10 rats. Even though the smaller amounts of the adjuvants usually used (0.25 ml) never produced proteinuria, slight histological alterations of the glomerular structure were frequently noted. 3) In contradistinction to other tissue suspensions addition of rat kidney supernate enhanced the effect of adjuvants markedly and regularly. 4) The small amounts of rat kidney proteins used, support the interpretation that an isoand autosensitization mechanism is at play.