The azopyrimidine, 6-(p-hydroxyphenylazo)-uracil, inhibits the replication of bacterial DNA selectively, completely, and reversibly, and has no apparent effects on the metabolism of other cellular macromolecules thus far examined. The mechanism of its action has been investigated in uninfected and phage-infected Bacillus subtilis, and the compound appears to be specific for a host function. In cells infected with virulent phage the synthesis of phage DNA proceeds normally, while residual host DNA synthesis is completely blocked. The drug-sensitive host site retains its sensitivity even after partial disruption of the cell by lysozyme treatment.