The DNA-breaking and -joining steps initiating retroviral integration are well understood, but the later steps, thought to be carried out by cellular DNA repair enzymes, have not been fully characterized. Poly(ADP-ribose) polymerase 1 (PARP-1) has been proposed to play a role late during retroviral integration, because infection by human immunodeficiency virus (HIV)-based vectors was reported to be strongly inhibited in PARP-1-deficient fibroblasts. PARP-1, a nuclear enzyme, binds tightly to nicked DNA and synthesizes poly(ADP-ribose) as an early response to DNA damage. To investigate the role of PARP-1 in retroviral integration, we infected wild-type and PARP-1-deficient mouse embryonic fibroblasts (MEFs) separately with two HIV type 1-derived, vesicular stomatitis virus G-pseudotyped lentivirus vectors. Surprisingly, infection of both wild-type and PARP-1-deficient cells was observed with both vectors. Marker gene transduction and provirus formation by one vector was reduced by 45 to 75% compared to the wild type, but the other vector was unaffected by the PARP-1 mutant. In addition, PARP-1-deficient MEFs infected with Moloney murine leukemia virus showed no decrease in virus output after infection compared to the wild type. We conclude that PARP-1 cannot be strictly required for retroviral infection because replication steps, including integration, can proceed efficiently in its absence.