The level of alkaline phosphatase in a number of established cell lines of human origin can be modified by exposure to non-lethal concentrations of bromodeoxyuridine (BRdU). In the several cell lines examined an inverse relationship between amount of induction and constitutive level of the enzyme was observed. Thus, the H.Ep 2 line, which had the highest basal level of enzyme, was reversibly repressed following exposure to the drug, whereas other cell lines with relatively low constitutive enzyme levels were induced to a maximum of 10-fold following exposure. Initiation of induction required from 24 to 48 hours, and as short an exposure ("pulse") as five hours was sufficient to produce induction. Exposure to visible light had no effect upon the repression of alkaline phosphatase in H.Ep 2 by BRdU. Induction did not occur in non-dividing, serum starved cells. The time course of induction by BRdU and hydrocortisone was similar, and simultaneous exposure of the cells to both agents resulted in no greater induction than that observed with either drug alone. Experiments utilizing mitomycin C yielded significant induction in the presence of this agent alone, and somewhat less induction when both mitomycin C and BRdU were added simultaneously. These results suggest that DNA synthesis is required for BRdU were added simultaneously. These results suggest that DNA synthesis is required for BRdU-mediated induction of alkaline phosphatase.