Growth of CHRC5 multidrug resistant cells in media enriched in a saturated C-17 fatty acid, heptadecanoic acid, resulted in these cells accumulating vinblastine at a rate and to an extent comparable to that of the parental cell line AB1. The fatty acid-enriched growth media had no effect on the ability of AB1 cells to take up vinblastine. The action of amphiphiles on the uptake of rhodamine dyes by CHRC5 cells was compared with the increased dye accumulation affected by verapamil. Membrane rigidifying agents, such as the saturated fatty acid stearic acid, or the cholesterol derivatives, cholesteryl hemisuccinate and cholesteryl phosphorylcholine, as well as a membrane fluidizing unsaturated fatty acid, linoleic acid, could significantly increase dye uptake, although not as well as verapamil. These results taken in conjunction with other reports in the literature, demonstrate that multidrug resistance is sensitive to alterations of membrane properties. They suggest that perturbation of the membrane to either increased or to decreased membrane fluidity can lower the level of resistance.