Two of the flower extracts have been further investigated in drug-sensitive, P-glycoprotein-negative CCRF-CEM leukemia cells and multidrug-resistant, P-glycoprotein-expressing CEM/ADR5000 cells. We were interested to see whether the extracts were able to modulate the resistance of CEM/ADR5000 cells to the anticancer drug doxorubicin. As positive control, we used verapamil, which is a well-known inhibitor of P-glycoprotein and modulator of multidrug resistance (Tsuruo et al., 1981). As shown in Fig. 2, the n-hexane extract of Mesua ferrea and the dichloromethane extract of Michelia alba did partially reduce doxorubicin resistance in CEM/ADR5000 cells, whereas no effect was observed in CCRF-CEM cells. For comparison, verapamil also partially reversed doxorubicin resistance in CEM/ADR5000. While CEM/ADR5000 cells were more than 5000-fold resistant to doxorubicin compared to CCRF-CEM cells, the addition of verapamil did not reverse doxorubicin resistance of CEM/ADR5000 cells to the level of the CCRF-CEM cells. CEM/ADR5000 cells were still more than 200-fold resistant to doxorubicin after co-treatment with verapamil compared to CCRF-CEM cells.