The mechanisms responsible for the antiproliferative and cytotoxic effects of the anthracycline antibiotics doxorubicin (Adriamycin) and daunorubicin (daunomycin) have been the subject of considerable controversy. This commentary addresses the potential role of DNA synthesis inhibition, free radical formation and lipid peroxidation, DNA binding and alkylation, DNA cross-linking, interference with DNA strand separation and helicase activity, direct membrane effects, and the initiation of DNA damage via the inhibition of topoisomerase II in the interaction of these drugs with the tumor cell. One premise underlying this analysis is that only studies utilizing drug concentrations that reflect the plasma levels in the patient after either bolus administration or continuous infusion are considered to reflect the basis for drug action in the clinic. The role of free radicals in anthracycline cardiotoxicity is also discussed.