Deoxycytidine kinase (dCyd kinase) is important for the phosphorylation of several different nucleoside antimetabolites. To understand the significance of dCyd kinase levels in chemotherapy, dCyd kinase mRNA levels were evaluated in several cells with a quantitative competitive polymerase chain reaction (PCR) assay. dCyd kinase catalytic activity and intracellular ara-CTP production were also compared with the levels of dCyd kinase mRNA. The assay was able to show: (i) that dCyd kinase catalytic activity and dCyd kinase mRNA levels were correlated in cells; (ii) that dCyd kinase mRNA levels were more sensitive in lower levels of 10 amol/micrograms of total RNA; and (iii) in cytosine arabinoside (ara-C)-resistant cells, both dCyd kinase mRNA levels and intracellular ara-CTP levels were lower compared with levels in sensitive cells. The PCR assay for dCyd kinase mRNA could be useful in the selection and monitoring of patients treated with nucleosides that are activated by this enzyme.