1. The therapeutic action of an anticomplementary agent, fumaropimaric Acid (FMP), was investigated in experimental immune nephritis. 2. Nephrotoxic glomerulonephritis of variable severity was produced in rats with intravenous antiglomerular antiserum. Four to 20 hours later 51Cr. EDTA clearances, protein excretion and urinary flow rates were studied by ureteric catheterization during mannitol diuresis. 3. The action of FMP was assessed under these conditions. It was intravenously infused and serial changes in renal function in 15 nephritic and 5 control animals analysed. 4. Four of the nephritic group showed a degree of improvement related to the treatment period. Seven animals responded with statistically insignificant improvement in renal function. 5. Histological examination suggested that FMP may have reduced the number of polymorphonuclear leucocytes in the nephritic glomeruli.