DNA synthesis has been studied in an in vitro system derived from cells infected with bacteriophage T4. This in vitro system consists of cells treated with toluene. Such cells, though no longer viable, nevertheless incorporate exogenous deoxyribonucleotide triphosphates into DNA (Moses and Richardson, 1970; Mordoh, et. al., 1970). Toluenization of T4-infected cells results in the reversal of phage-induced shutoff of host DNA synthesis. To see this result, however, the infecting phage must carry a mutation in the D2a region of the T4 genome. If the phage is D2a+ host DNA synthesis in vitro is severely depressed. Thus, the D2a product appears to inhibit, either directly or indirectly, bacterial DNA synthesis in toluenized cells. The D2a product probably controls (or is) a cytosine-specific deoxyribonuclease (Bruner, et. al., 1972). T4 DNA synthesis can occur in toluenized cells. T4 mutants which are defective in host DNA degradation nevertheless shut off host DNA synthesis in vivo.