[en] To investigate whether a delay in the rejoining of radiation-induced strand breakage can lead to sister chromatid exchange formation, Chinese hamster ovary cells were prelabeled with 5-bromodeoxyuridine and X-irradiated in the presence of 3-aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase. The resulting sister chromatid exhange frequencies were consistent with those expected if 3-aminobenzamide and X-ray treatments were independent and additive. A similar but much smaller additive effect was also observed in cells cultured in the presence of 3-aminobenzamide and X-irradiated immediately before the addition of bromodeoxyuridine to the culture medium. These findings support previous studies indicating that X rays are poor inducers of sister chromatid exhanges and suggest that the normally rapid resealing of DNA strand breaks does not account for this inefficiency