AbstractIn this study, the relationship between the dCTP/ara-CTP ratio and the cytotoxic effect of cytosine arabinoside (ara-C) was investigated. Intracellular levels of dNTPs and ara-CTP were analyzed by high-performance liquid chromatography. Although doubling time and intracellular dCTP levels were different in each of the cells, there was a consistent relation between the intracellular dCTP/ara-CTP ratio and the cytotoxic concentration of ara-C. These data suggest that the intracellular dCTP/ara-CTP ratio is one of the important factors for considering the cytotoxic action of ara-C.