1. A strain of S. muscae which requires a substance present in certain acid-hydrolyzed proteins (AHPF) for virus liberation when singly infected in Fildes' synthetic medium no longer needs this substance when multiply infected.
2. In the absence of the AHPF under conditions of multiple infection the amount of phage released is approximately equal to the number of infecting particles between two to ten. Over ten particles per cell has no further effect on the yield of virus.
3. The experimental evidence indicates that it is the phage particle and not some other component in the lysate which can replace the AHPF.
4. The minimum latent period and rise period of cells singly infected in the presence of the AHPF and multiply infected in the absence of the AHPF are the same.
5. The desoxynucleic acid synthesis of cells, infected with a very few virus particles in the presence of excess AHPF and multiply infected with ten particles in the absence of the AHPF, occurs at approximately the same rate, with both infected samples synthesizing about the same amount of desoxynucleic acid and liberating the same yields of virus.
6. A strain of S. muscae which requires aspartic acid for virus synthesis when singly infected does not need this substance when multiply infected, the burst size under the latter conditions depending upon the multiplicity of infection between 3 to 12 particles per cell.
7. The data indicate that the virus released from multiply infected cells in the absence of added AHPF or aspartic acid is newly synthesized virus and not the original infecting particles.
8. The phage particle contains the AHPF and aspartic acid.
9. As a tentative working hypothesis, it is assumed that the AHPF and aspartic acid for phage formation under conditions of multiple infection, in the absence of added AHPF, or of aspartic acid, are contributed by the original infecting particles.
10. Ultraviolet-inactivated phage is adsorbed to the host cell and kills the cell although little virus is released under the experimental conditions.
11. Ultraviolet-inactivated phage particles, if added before the active particle is adsorbed, will greatly inhibit the liberation of new virus particles; but does not do so if added a few minutes after the active particle has been adsorbed.
12. Under the experimental conditions, reactivation of phage when present in multiply infected cells does not occur; and such ultraviolet-inactivated phage cannot serve as a source of the AHPF or aspartic acid, although the AHPF can be liberated from such inactivated particles by acid hydrolysis.
13. The results are discussed in relation to Luria's experiments with ultraviolet-treated phage and to his "gene pool" hypothesis of phage formation.