The irradiation ofEscherichia coliB with sublethal doses of 3000–4000 Å light prevented the microorganisms from manufacturing β-galactosidase and T2 and T7 coliphages. Inhibition occurred only if the cells were irradiated immediately after their contact with the inducer lactose or infection with T2 and T7 phages. If, before irradiation the cells were allowed to incubate for 15 min after the addition of lactose or the coliphages to the cells, little effect of the light was found. The uptake of uracil and amino acids by washed cells was more rapid in the first 15 min than during later time periods while thymine uptake did not begin until the first 15 min had elapsed. The 3000–4000 Å light inhibited the uptake of arginine and thymine but not uracil or glutamic acid. The addition of 5% inositol inhibited the synthesis of β-galactosidase and the uptake of14C-labelled metabolites. Since there was a strong correlation between the degree to which arginine and thymine uptakes were inhibited by the light or inositol, it appears that the production of a protein during the first 15 min is intimately connected with DNA replication and the synthesis of induced enzymes.