In vitro studies were performed on the interaction of indomethacin with cholestyramine, a hypocholesterolemic substance. Cholestyramine showed a marked affinity for indomethacin among tested acidic drugs and the intensity of adsorption was dependent on pH, temperature and sodium chloride. Moreover, the combination of indomethacin with some acidic drugs that formed complexes with cholestyramine, considerably inhibited the adsorption of indomethacin on the resin.