1 Multidrug resistance (MDR) to antitumour agents, structurally dissimilar and having di erent intracellular targets, is the major problem in cancer therapy. MDR phenomenon is associated with the presence of membrane proteins which belong to the ATP-binding cassette family transporters responsible for the active drug e ux leading to the decreased intracellular accumulation. 2 The search of new compounds able to overcome MDR is of prime importance. 3 Recently we have synthesized a new family of anthrapyridone compounds. The series contained derivatives modi®ed with appropriate hydrophobic or hydrophylic substituents at the side chain. 4 The interaction of these derivatives with erythroleukemia K562 sensitive and K562/DOX resistant (overexpressing P-glycoprotein) cell lines has been examined. The study was performed using a spectro¯uorometric method which allows to continuously follow the uptake and e ux of uorescent molecules by living cells. 5 It was demonstrated that the increase in the lipophilicity of anthrapyridones favoured the very fast cellular uptake exceeding the rate of P-gp dependent e ux out of the cell. For these derivatives, very high accumulation (the same for sensitive and resistant cells) was observed and the in vitro biological data con®rmed that these compounds exhibited comparable cytotoxic activity towards sensitive and P-gp resistant cell line. In contrast, anthrapyridones modi®ed with hydrophylic substituents exhibited relatively low kinetics of cellular uptake. 6 For these derivatives decreased accumulation in resistant cells was observed and the in vitro biological data demonstrated that they were much less active against P-gp resistant cells in comparison to sensitive cells. 7 We also studied, using confocal microscopy, the intracellular distribution of anthrapyridones in NIH-3T3 cells. Our data showed that these compounds were strongly accumulated in the nucleus and lysosomes.