The growth of differentiated cell monolayers on microporous filters is providing powerful new techniques for investigating the transport of drug and delivery systems across defined cellular barriers, and for discriminating between different routes and mechanisms. The growth, characterization and potential use of these systems is illustrated by studies on the human Caco-2 cell system which provides an in vitro model of the intestinal epithelial barrier. This system, still in the early stages of characterization and development, displays a number of carrier-mediated and vesicular transport systems found in the intestine in vivo, and is thus providing a useful system for studying the intestinal transport of drugs including peptides and proteins.