A significant proportion of P-glycoprotein (P-gp) and caveolin was co-localized in caveolae isolated from resistant (CH(R)C5) cells overexpressing P-gp and from drug-sensitive Chinese hamster ovary cells (AuxB1). The proportion of P-gp and caveolin associated with caveolar microdomains was higher in CH(R)C5 cells grown in the presence of P-gp substrates (cyclosporin A or colchicine) than in untreated CH(R)C5 cells. Coimmunoprecipitation of P-gp and caveolin from CH(R)C5 lysates suggests that there is a physical interaction between them. Furthermore, co-localization of P-gp and caveolin was found in caveolae from brain capillaries, indicating that this association also takes place in vivo.