1. Four strains of Staphylococcus muscae have been isolated which differ in their growth rates and phage syntheses in Fildes' synthetic medium.

2. Two of the strains when singly infected cannot release phage in Fildes' synthetic medium unless a substance present in certain acid-hydrolyzed proteins is added to the medium. One of these strains also requires other substance(s) present in acid-hydrolyzed proteins in order to grow in Fildes' medium.

3. The two strains which do not require the addition of the phage-stimulating factor have been found either to synthesize this substance, or one similar to it. One of these strains will not grow in Fildes' medium unless substance(s) present in acid-hydrolyzed proteins is added to the medium.

4. The purified acid-hydrolyzed protein factor necessary for virus liberation does not affect the multiplication rate of uninfected S. muscae cells in Fildes' synthetic medium.

5. The substance is not needed for the adsorption or the invasion of the host cell by the virus. In the absence of the factor, the virus is adsorbed to the cell and "kills" it.

6. An analysis carried out by means of the one-step growth curve technique has indicated that the substance is not concerned simply with the mechanism of virus release, but is necessary for some initial stage in virus synthesis.

7. With one bacterial strain not requiring the AHPF, aspartic acid had to be present at least during the minimum latent period for the cell to form virus.

8. In the absence of aspartic acid, the virus was adsorbed to the cell and killed it, but no virus was released from singly infected bacteria.

9. If the cells were grown in a medium containing aspartic acid and then resuspended in the medium minus aspartic acid, no virus was released, although such cells contained at least two times the amount of aspartic acid necessary for the burst size in the complete medium.

10. Aspartic acid, a constituent of the virus particle, appears from an analysis of one-step growth curves to take part in the initial phase of phage synthesis.

11. The effect of amino acids on virus formation is discussed in relation to the time sequence of virus protein and desoxyribonucleic acid synthesis.

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