Johnson, R. C. (Fort Detrick, Frederick, Md.) and N. D. Gary. Nutrition of Leptospira pomona. I. A chemically defined substitute for rabbit serum ultrafiltrate. J. Bacteriol. 83:668–672. 1962.—Poor growth of Leptospira pomona, strain Wickard, in a medium containing exhaustively dialyzed rabbit serum was corrected by addition of rabbit serum ultrafiltrate. A dialyzed serum medium was used to investigate the nutritional qualities of various compounds when tested as substitutes for the ultrafiltrate fraction. l-Asparagine was the only amino acid that markedly stimulated growth. l-Glutamine was active only when autoclaved (converted to the ammonium salt of pyrrolidone carboxylic acid), and NH4Cl and urea satisfactorily replaced l-asparagine. Results indicated that amino acids functioned primarily as a nitrogen source in this medium.