Deoxycytidine kinase (dCK) phosphorylates deoxycytidine, deoxyguanosine, and deoxyadenosine and plays an important role in the salvage pathway of nucleoside metabolism. dCK is also required for the phosphorylation of several antiviral and anticancer nucleoside drugs, with resistance to these agents often being associated with a loss or decrease in dCK activity. Data also indicate a role for dCK in immune function, and dCK inhibitors may provide treatment for immune disorders. To identify novel dCK inhibitors, the authors evaluated 2 existing biochemical assays, adapted both to high-throughput screening, and identified several series of hits. They also compared the potency of the hits between purified recombinant and endogenous enzyme. Meanwhile, they also developed a novel cell-based assay that rests on the rescue of cells from dCK-dependent cytotoxic agents such as AraC. A large number of compounds were tested using the 3 assays, and a strong correlation in potency was observed between the biochemical assay using endogenous enzyme and the cell-based assay. The hits identified in these screens have proved to be good starting points for the synthesis of much more potent tool compounds to further investigate the physiological functions of dCK and potentially lead to the development of therapeutic agents.