The effect of 24 newly synthesized quinoline derivatives on tumor cell multidrug resistance (MDR) was examined in vitro. At low concentrations, these compounds enhanced the accumulation of [3H]vincristine in K562/ADM cells and reversed tumor cell MDR. The results of the structure-activity relationship analysis indicate that in highly active compounds the two aryl rings in the hydrophobic moiety deviate from a common plane, so they are capable of interacting with hydrogen bond donors of P-170 glycoprotein (P-gp) via pi-hydrogen-pi interactions. Other major structural features which influence the MDR-reversing activities of these compounds are a quinoline nitrogen atom and a basic nitrogen atom in piperazine. Furthermore, in highly active compounds, the distance between the hydrophobic moiety and the basic nitrogen atom (an atom connected to 2-hydroxypropoxyquinoline) must be at least 5 A. Several compounds were found to reverse vincristine resistance in K562/ADM cells in vitro, and compound 16 (MS-209) was selected for clinical studies.