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Wild-type bacteria which restrict the deoxyribonucleic acid (DNA) of infecting phage when the phage do not carry the proper host modification rapidly degrade that restricted DNA to acid-soluble products. The purified restriction enzyme acts as an endonuclease in vitro to cleave restrictable DNA and does not further degrade the DNA fragments produced. We have examined mutants of Escherichia coli K-12 which lack various nucleases in order to determine which nucleases are involved in the rapid acid solubilization in vivo of unmodified A DNA following restriction. Bacteria which are wild type, recA-, or polAl-degrade about 50% of the unmodified phage DNA within 10 min of infection, with little subsequent degradation. Mutants which are recBor recCdegrade unmodified DNA very slowly, solubilizing about 15% of the DNA by 10 min after infection. Two classes of phenotypic revertants of recB-/Cmutants were also tested. Bacteria which are sbcA-restrict poorly and do not degrade much of the restricted DNA. Bacteria which are sbcBrestrict normally. This mutation does not appear to affect degradation of restricted phage DNA in recB-/Cmutants, but such degradation is decreased in recB+/C+ bacteria. The presence of a functional A exonuclease gene is not required for degradation after restriction.