One of the most specific functions of the endothelial layer is to act as a selective barrier between blood/lymph and tissues. A standardized assay able to measure permeability of in vitro cultured endothelial cells is therefore highly desirable. In principle such an assay measures the passage of an easily detectable tracer between two compartments separated by an endothelial layer. The assay of trans-monolayer electrical resistance, which measures the passage of ions through the cell layer, while very effective for testing barrier properties of epithelial cells which form very tight layers in culture (Madara 1998), is less suitable for endothelial cells. With the exception of arterial or brain microvascular endothelium, most types of endothelial cells in vitro form layers that present low electrical resistance, which makes it difficult to pick up a further decrease.