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Fresh specimens of human peritoneum collected from heart-beating cadaver organ donors have been examined by transmission electron microscopy. Samples were taken from the anterior abdominal wall and from the surfaces of the liver, stomach and diaphragm. The mesothelium consisted of a single layer of flattened cells generally 2.5 microns to 3 microns thick. These were joined by tight junctions and desmosomes to form a continuous sheet. The cells rested on a prominent basement membrane deep to which was a layer of fibrous connective tissue. This layer was more compact under the mesothelium from the abdominal wall and liver than elsewhere. Long microvilli projected from apical surface of the cells. In many cases these covered the entire surface but sometimes they were more profuse at the edges of the cells near the intercellular junctions. The cells possessed a well-developed cytoskeleton of intermediate filaments which coursed through the cytoplasm in thick bundles. The cells also had a well-developed rough endoplasmic reticulum and Golgi apparatus. Numerous smooth-surfaced and coated vesicles could be seen adjacent to the plasmalemma at all surfaces, providing evidence of considerable pinocytotic activity. There was little regional variation in the structure of the mesothelium. We found no evidence of pores passing through the layer although, on the liver, cisternae were present between the cells and these were often occupied by lymphocytes.