Renal glomerular lesions were induced by rabbit serum containing antibody to rat collagen injected intravenously into rats prepared with subcutaneously administered Freund adjuvant. Neither the anti-collagen serum nor the adjuvant alone induced the lesion. The lesions were characterized by diffuse glomerular injury with swelling, shredding, and fusion of the basement membranes, crescent formation, cellular proliferation, numerous multinuclear giant cells, and capillary hyaline thrombi. Various rabbit antisera, including those against fish collagen or rat serum failed to induce the renal lesion when substituted for anti-rat collagen serum. Also, anti-rat collagen serum absorbed with its homologous antigen, native rat collagen, failed to induce the lesion. Although complete adjuvant, i.e. with mycobacteria, in which normal serum was incorporated enhanced the glomerular lesion which resulted from intravenous injection of anti-collagen serum, the incomplete adjuvant without serum was sufficient. Comparison of the renal lesions induced by anti-collagen serum with nephrotoxic nephritis induced in rats by rabbit anti-kidney serum showed that they differ histologically. Also the antisera used to produce these two renal lesions differ immunologically. Antibodies to normal rabbit serum developed in rats injected intravenously with rabbit anti-rat collagen serum after preparation with adjuvant, but not when adjuvant was omitted. The pathogenesis of the renal injury is discussed as a manifestation of an antigen-antibody reaction, with nephritis occurring only after the adjuvant-stimulated antibody to the rabbit globulin has been formed in the rat and has reacted with the rabbit anti-rat collagen already fixed by its homologous antigen in the kidney.