The process of genome DNA injection, after adsorption, by phage PL-1 into Lactobacillus casei ATCC 27092 was monitored by electron microscopy. The DNA injection depended on the incubation temperature, and the apparent activation energy was about 11 kcal. It was inhibited when the cells had been previously starved, where their intracellular ATP contents was lowered less than one-hundredth that of the unstarved cells. There was a good correlation between the ATP contents of cells and the extent of the phage DNA injection. Dicyclohexyl carbodiimide inhibited the process with little effect both on the viability of cells and the infectivity of phages. These results agreed with the view that a high energy level of the host cells would be required for the formation of blender-resistant phage-cell complexes to complete injection of phage DNA into host cells.