An alkaloidal extract of the vines of Stephania japonica showed multidrug-resistance-reversing activity as demonstrated by the bicinchoninic acid assay. Two known bisbenzylisoquinoline alkaloids, isotrilobine (1) and trilobine (2), were isolated by bioassay-directed fractionation and separation. Isotrilobine (1) was shown to be as active as verapamil (3) in reversing doxorubicin resistance in human breast cancer cells.