A chemically defined medium made by adding sodium acetate and Tween 80 (a source of oleic acid) to the medium of Niven (1944) permitted the growth of all strains of the lactic group of streptococci which dial not grow on the unsupplemented medium. The addition of sodium acetate and Tween 80 was necessary for the growth of 22 strains of S. cremoris and 9 of 31 strains of S. lactis. Reticulogen, a commercial liver extract, could be substituted in somewhat smaller quantities for sodium acetate and Tween 80 and also permitted rapid growth of one strain of S. cremoris which did not show detectable growth until after 24 hours in the medium supplemented with sodium acetate and Tween 80;Using ammonia formation from arginine and growth at 40°C as the basis for separation, all strains of the lactic group which had been in the laboratory for considerable time were found to be S. cremoris, while all recently isolated strains were found to be S. lactis;With two S. lactis-bacteriophage combinations, multiplication of bacteriophages and organisms were affected similarly by the omission of individual components from the unsupplemented synthetic medium of Niven (1944). Bacteriophage multiplication seems to be closely associated with organism multiplication for these two combinations;When calcium was available in the medium, eight bacteriophage strains tested were found to multiply on their susceptible host cells in a completely synthetic medium which did not permit these bacteriopage strain to proliferate without the addition of CaCl2·2HOH. When calcium was available in the medium and these bacteriophages multiplied, the close relationship between bacteriophage multiplication and organism multiplication seemed evident for these strains also. Calcium was rendered unavailable in a medium containing 0.1 percent CaCl2·2HOH by either autoclaving the entire medium or by increasing the K2HPO4 content above 0.1 percent;A close relationship seems to exist between the nutrition or organisms of the S. lactis group and multiplication of their homologous bacteriophages. However, calcium, while of no detectable importance to organism growth, seems to be required for the multiplication of many bacteriophages.