A new approach to the measurement of intestinal permeability in man has been developed, using low molecular weight polyethylene glycol (PEG 400) as probe molecules. PEG 400 (range of molecular weight 232 to 594) is a mixture of water-soluble molecules of different sizes which can be extracted readily from biological fluids and analyzed by gas-liquid chromatography. PEG 400 is nontoxic, not degraded by intestinal bacteria, not metabolized after absorption, and rapidly excreted in urine. The different-sized molecular components cross the intestinal epithelium at different rates, allowing characterization of the passive permeability properties of the mucosa.