Experiments were carried out in vitro using DNA polymerase and ribonucleotide reductase inhibitors to investigate their cytotoxicity to P388 murine leukemia sensitive (P388/S) and resistant (P388/R) to adriamycin (ADR). DNA polymerase inhibitors such as cytosine arabinoside (ara-C) and aphidicolin elicited comparative inhibition of DNA biosynthesis in both parental and ADR-resistant tumor cells. However, ribonucleotide reductase inhibitors such as hydroxyurea (HU) and caracemide were collaterally more sensitive to P388/R cells. Inosine diglycolaldehyde (Inox) was ineffective in showing such a response. Pretreatment with HU significantly increased intracellular ADR levels and inhibition of RNA biosynthesis by ADR in P388/R cells while, in P388/S cells, sequential or concurrent treatment with HU did not enhance intracellular ADR levels. Mechanisms underlying such an effect, implications due to reduced intracellular ATP levels in drug-resistant cells, and the possible utility of using ribonucleotide reductase as a target in drug-resistant tumors for the therapeutic benefit are discussed.