Abstract The addition of deoxyribonucleic acid from the cells of a lactose-fermenting strain (Hfr, constitutive) to the reaction mixture containing the particle and the soluble fraction from a lactose-nonfermenting strain (F), caused some increase in β-galactosidase activity. The increase in β-galactosidase activity was entirely dependent upon the presence of gene-specific deoxyribonucleic acid, energy source, cell-free preparations containing the particle and the soluble fraction, except for the incomplete dependence upon nucleotides and an inducer. And the reaction was completely prevented by chloramphenicol, amino acid analogue or ribonuclease. In parallel with the increase in enzyme activity, the incorporation of 14C-1- dl-leucine into the protein precipitable with anti-sera to β-galactosidase was found, which was dependent upon biologically active deoxyribonucleic acid and an inducer. Deoxyribonucleic acid prepared from lactose-nonfermenting strain and DNase-treated or UV-irradiated DNA were inactive in the above reaction. In contrast the digestion of deoxyribonucleic acid by ribonuclease or trypsin influenced hardly any its activity.