The process of genome DNA injection, after adsorption, by phage PL-1 into host cells of Lactobacillus casei was monitored by using the electron microscope. Injection of DNA was inhibited by the protein-synthesis inhibitors chloramphenicol and erythromycin at concentrations where the colony-forming ability of cells not infected by phage was unaffected. The results suggest that protein synthesis may be involved in some way in the process of genome injection.