1. The addition of penicillin greatly increases the production of phage in bacterial suspensions containing 2.5 to 3.5 x 108 cells in 0.4 ml. broth plus 6.6 ml. Locke's solution.
2. Addition of niacin also greatly increases the formation of phage in the above system without the addition of penicillin.
3. The results indicate that niacin is necessary for phage production and that bacteria cannot utilize niacin in the presence of penicillin.
4. Staphylococcus muscae will grow in the synthetic medium of Fildes but do not form phage unless broth or yeast extract is added.
5. Phage formation requires the presence of one or more factors, besides niacin, present in broth and yeast extract which are not essential for bacterial growth. Penicillin does not prevent the utilization of the unknown substance or substances by the bacteria.
6. A solution containing biotin, guanine, adenine, ß-alanine, riboflavin, uracil, pyridoxamine, guanylic acid, adenylic acid, yeast nucleic acid, choline, p-aminobenzoic acid, a flavin component from liver, ribose, thymine, xanthine, folic acid, inositol, p-aminophenyl alanine, pantothentic acid and a strepogenin concentrate cannot replace broth or yeast extraction in increasing phage formation in the synthetic medium of Fildes.
7. The results indicate there is a continual competition between the bacteria and phage for certain essential building elements.
8. The results are discussed in relation to possible methods of control of virus diseases.