Topoisomerases are nuclear enzymes that maintain and modulate DNA structure. Inhibitors of topoisomerases like camptothecin (CPT), etoposide, and others are widely used antitumor drugs that interfere with transcription, induce DNA strand breaks, and trigger apoptosis preferentially in dividing cells. Because transcription inhibitors (actinomycin D, galactosamine, ␣-amanitin) sensitize primary hepatocytes to the cytotoxic action of tumor necrosis factor (TNF), we reasoned whether topoisomerase inhibitors would act similarly. CPT alone was not toxic to primary cultured murine hepatocytes. When incubated with CPT, murine hepatocytes displayed an inhibition of protein synthesis and were thereby rendered sensitive to apoptosis induction by TNF. Apoptosis was characterized by morphology (condensed/fragmented nuclei, membrane blebbing), caspase-3-like protease activity, fragmentation of nuclear DNA, and late cytolysis. T opoisomerases are important nuclear enzymes that regulate DNA tertiary structure by mediating transient, protein-shielded DNA breaks. Topoisomerases are involved in DNA replication, repair, recombination, and in transcriptional regulation. Thus, their activity is particularly required during mitosis to facilitate DNA re-organisation. Type I topoisomerases cut and pass single strands of DNA, whereas type II enzymes cut and pass double-stranded DNA.Topoisomerase II inhibitors (etoposide [ETP], doxorubicin) and the topoisomerase I inhibitor camptothecin (CPT) and derivatives are in clinical use or clinical trials as chemotherapeutics. 1 By covalently trapping topoisomerases on DNA, the compounds preferentially target dividing cells that require high topoisomerase activity. Under topoisomerase inhibition, DNA strand breaks occur during mitosis, which triggers premature termination of replication and inhibits