THE FORMATION of growth-promoting substance (auxin) by microorganisms has been observed in recent years by a number of investigators, but comparatively little knowledge exists concerning the relationship between definite compounds present in the substrate and elaboration of these special substances. In this study pure cultures of Aerobacter aerogenes and Escherichia coli were grown in synthetic media containing single amino acids2 or inorganic salts as the only source of nitrogen. After periods of incubation, the cultures were tested for growth substance according to a standard Avena coleoptile curvature method commonly used for detection of phytohormones causing cell enlargement. METHODS.-The culture medium employed for growing the bacteria was glycerol agar made up according to the formula used by Koser and Rettger (1919). Its constitution is as follows: NaCi, 5.0 g.; MgSO4, 0.2 g.; CaCl2, 0.1 g.; KH2PO4, 1.0 g.; K2HPO4, 1.0 g.; glycerol, 30.0 g.; distilled water, 1,000 g. Sufficient agar was added to make a 2 per cent mixture. Nitrogen was supplied to the cultures by adding a single amino acid to the substrate at the rate of 1 gram per 1,000 cc. of medium. In experiments with inorganic sources of nitrogen the composition of the medium was similar, but 1 gram of NH4Cl or of KNO3 was used per liter, and 4.0 grams of NaCl were added instead of 5.0 grams per liter. All media were put into pyrex culture tubes and sterilized in an autoclave. Only 2 cc. of medium were used in each tube, and the agar was allowed to solidify in an extremely slanted position, so as to permit considerable aeration of the culture during the period of growth. The tubes were inoculated from ordinary nutrient agar slant cultures of A. aerogenes and E. coli, using a small amount of inoculum. The inoculated tubes were kept in a moist incuw bator at 33.0?C. for about two or four weeks. Sterile tubes of media were held as controls along with the cultures. During the period of incubation, some of the cultures developed pronounced colors and heavy growths of bacteria, while others remained less colored or showed less growth. At the time selected for making tests for growth substance, agar slants containing the bacteria as well as sterile control tubes of the same media were melted in a water bath, and from each sample agar plates were cast in a brass mold. Great care was exercised in keeping all preparations clean and free from contamination during the testing procedure. The standard size