In bacterial strains containing the deoxyribonuclease endonuclease I (endonuclease I(+) strains), 70 to 80% of the injected superinfecting T-even phage deoxyribonucleic acid (DNA) is rapidly degraded to oligonucleotides having an average chain length of 8, the same value as that obtained by endonuclease I digestion of purified T-even phage DNA in vitro. In endonuclease I(-) strains, less than 5% of the injected superinfecting T-even phage DNA is degraded to acid-soluble components. The superinfecting phage DNA is, however, fragmented into a large segment having a molecular weight of about 90 x 10(6) and 30 or more small acid-insoluble segments having molecular weights of less than 10(6). In both endonuclease I(+) and endonuclease I(-) strains, over 80% of the DNA from adsorbed primary T2 or T4 phage, but only 50% of the DNA from adsorbed superinfecting T2 or T4 phage, is injected. Superinfecting T4 are genetically excluded as efficiently from endonuclease I(-) strains as they are from endonuclease I(+) strains. The excluded phage cannot complement defects in either early or late gene functions carried by the primary phage. The induction of both superinfection breakdown and superinfection exclusion requires a period of protein synthesis between primary infection and addition of the superinfecting phage. These observations seem best explained by failure of superinfecting DNA to enter the host cell cytoplasm, presumably as a result of changes in the cell envelope induced by the primary phage.