Of the compounds that have now been shown to circumvent acquired cellular multidrug resistance, little or no structure-activity relationship has been found, although their proposed mechanism of action is through modulation of function of p-glycoprotein. While it has been suggested that this inhibition is a direct binding to p-glycoprotein, we show here that such a model seriously neglects the effects many of these compounds have on lipid physical properties. We have characterized the interactions between 16 structurally diverse pharmacological agents (nine of which are known to reverse multidrug resistance) and a variety of lipids. Potent modulators inhibit the membrane binding of rhodamine 6G, and we have observed a correlation of the measured Ki values with the effectiveness of the compounds in situ. We have determined the effects of the compounds on detergent micellization, and have shown substantial changes on the critical micelle concentration of detergents in the presence of modulators. Finally, we have examined the changes in model membrane 'viscosity' induced by the compounds. These results indicate that both direct p-glycoprotein and indirect lipid interactions of modulators should be considered in the mechanism by which these compounds reverse multidrug resistance.