The biodistribution of soluble macromolecules is governed extensively by their ability to penetrate endothelial layers. Many solid tumors possess vasculature that is hyperpermeable to macromolecules, not always correlating with the presence of interendothelial cell fenestrations. The exact physiological mechanisms responsible for this nonspecific leakiness are not yet fully understood. Together with enhanced vascular permeability, however, tumors usually lack effective lymphatic drainage; consequently, they selectively accumulate circulating macromolecules (up to 10% of an i.v. dose per gram in mice). This "enhanced permeability and retention effect" (EPR effect) has been studied extensively, and it is thought to constitute the mechanism of action of SMANCS (styrene-maleic/anhydride-neocarzinostatin), now in regular clinical use in Japan for the treatment of hepatoma. It seems likely that EPR also contributes to the anticancer activity of the N-(2-hydroxypropyl)methacrylamide copolymer-anthracycline conjugates which are shortly to undergo clinical evaluation in the U.K.