Probucol treatment in hypercholesterolemic patients promotes the transfer of cholesteryl esters from high-density-lipoproteins (HDL) to lower-density lipoproteins. In vitro studies, on hypercholesterolemic plasma incubated with or without probucol, show that drug binding to lipoprotein particles does not affect either cholesteryl ester transfer, or the lipoprotein modifications occurring in the process. Probucol stimulates reverse cholesteryl ester transfer in vivo apparently by mechanisms independent of lipoprotein binding.