Deoxycytidine kinase (dCK) is a rate-limiting enzyme for the activation of ara-C. It is responsible for the phosphorylation of ara-C which is widely used in the treatment of leukemia. We examined the effect of etoposide on dCK in L1210 cells and found that incubation with 10 microM etoposide for 1 h increased dCK activity to 170% of the control. This effect of etoposide on dCK activity was concentration-dependent up to at least 100 microM of the substance.