Plasma proteins interact with vascular endothelium in such a way as to render it less permeable to other macromolecules. Evidence from a variety of sources indicates that this may result from interaction of the circulating macromolecules with the negatively charged glycoprotein layer on the surface of endothelial cells, and that this layer may be responsible for some of the known molecular sieving properties attributed to the endothelium. Experiments with the fluorocarbon exchange-transfused rat are described, which suggest that there may be mechanisms other than vesicular translocation that facilitate the passage of macromolecules across endothelium. Such mechanisms include, among others, the formation of transient transendothelial channels that appear to be less sensitive than pinocytotic vesicles to the concentration of ambient protein. Recent evidence suggests that, in addition to molecular size and charge, glycosylation of protein molecules and cell membranes themselves may facilitate vesicular uptake.