When UV-irradiated T4 bacteriophage (v(+)) infects in the presence of chloramphenicol, the phage DNA rapidly acquires single-stranded breaks proportional to the dose of UV. In contrast, when UV-irradiated T4 v(1) (radiation sensitive mutant) infects under identical conditions, the phage DNA remains integral. A series of coinfections with v(+) and v(1) phage (UV-v(1) + majority non-UV-v(+) and UV-v(+) and majority non-UV-v(1)) show that the enzyme responsible for breakage is injected by the phage. It is also demonstrated that the v(1) phage injects an inactive enzyme that delays breakage by the v(+) enzyme and interferes with subsequent repair. The cross of v(+) and v(1) phage produces mixed progeny that contain both active and inactive enzyme in a single capsid. The possible function of this breaking enzyme, necessitating injection of multiple copies, is considered.