In 1942, the senior author reported that a strain of Streptomyces (Actynomyces) albus stimulated the growth of a strain of Trichophyton discoides Sabouraud, common cause of tinea in calves and children in Uruguay, on a glucose peptone agar. He also found that the addition of thiamine to the medium had the same effect. The ability of Streptomyces albus to synthesize thiamine was proved by the growth of Phycomyces blakesleeanus. Later on, Robbins, Mackinnon, and Ma (1942) reported that Mackinnon's strain of Trichophyton discoides had a ccmplete deficiency for pyridoxine, DLinositol, and molecular thiamine. In addition, there were partial deficiencies for unidentified substances present in pEptcne, casein hydrolyzate, hydrolyzed egg albumen, malt extract, gelatin, and a filtrate prepared frem white potatoes. These partial deficiencies could not be Eatisfied by biotin, riboflavin, folic acid, pimelic acid, para-aminobenzoic acid, pantothenic acid, hypoxanthine, guanine, nicotinamide, 2-methyl-1-1.4-naphthoquinone diacetate, seven pyrimidine and purine bases, and 43 amino acids. After our studies with strain 688, which we identified as T. discoides, we thought that vitamin deficiencies might be useful for the identification of species; that is, that strains of the same species would show the same deficiencies. Trying to elucidate this interesting problem we cultivated 5 strains of T. discoides and 2 strains of T. ochraceum on synthetic media with pyridoxine, thiamine, and DL-inositol, alone and in all possible combinations. EXPERIMENTAL STUDY Materials and methods. The organisms were cultivated at 30 C on agar slopes in neutral glass test tubes, 20 by 150 mm. Each tube contained 8 ml of a basal medium composed, per liter, of 50 g glucose, 2.0 g purified asparagine, 0.5 g MgSO4c7H2O, 1.5 g KH2PO4, and 15 g purified agar. To this medium 0.1 ml of the following solution of mineral supplements was added: 5.7 mg H3BO3, 18.6 mg CuSO4'5H20, 173 mg (NH4)2Fe2(SO4)4-24H20, 7.1 mg MnCl2-4H20, 3.6 mg ammonium molybdate (85 per cent), 79 mg ZnSO4c7H20, and 100 ml redistilled water. This medium was autoclaved for 20 minutes at 115 C. The growth factors were added before solidification of the medium in the dose of 5 millimicromoles of thiamine, 5 millimicromoles of pyridoxine, and 1 mg of DL-inositol, singly and in all possible combinations. At the same time tubes were prepared containing the basal medium plus 0.1 g peptone (Difco) and 5 millimicromoles of thiamine. To another series of tubes nothing was added (basal medium).