We have previously shown that very high levels of hypersensitivity to several resistance modifiers are correlated with increasing multidrug resistance in a series of Chinese hamster ovary cell lines. We have now selected a new member of the series which is an exception to this correlation in that although it is almost twice as multidrug resistant as the cell line from which it was derived, it shows much less hypersensitivity to resistance modifiers. Level of resistance modifier hypersensitivity correlated with the level of reduction of verapamil accumulation in these cells, and with the density of P-glycoprotein, but since the selection of this cell line has involved a doubling of cell volume, it was not correlated with total amount of P-glycoprotein.