DNA extracted from x-irradiated cells of a mutant of Bacillus subtilis deficient in DNA polymerase I has greatly reduced biological activity. This DNA, which has many single-strand nicks, can be repaired in vitro with purified DNA polymerase I and DNA ligase or with lysates of wild-type B. subtilis cells, which restore biological activity and increase the single-strand molecular weight. Lysates of polymerase I-deficient B. subtilis cells cannot repair such irradiated DNA until purified DNA polymerase I or lysate from wild-type cells is added to the deficient lysate.