This is a review of the literature on the lining cells of the peritoneal cavity from 3 points of view: their characteristics as lining cells and their relationship to the endothelium of blood vessels and fixed connective tissue cells; the work done on production of phagocytic mononuclear cells from meso-thelial cells and relation of the latter to formation of adhesions; and the role of the lining cells in absorption of solutions and particulate materials from the peritoneal cavity. It is concluded that the mesothelial cells represent a specific strain which are not interchangeable with connective tissue cells, which do not act as "Anlagen" for the production of phagocytic mono-nuclears, and which are the essential elements that prevent adhesions. For the most part they seem to represent a relatively inert, permeable membrane, but some evidence suggests an active role of minor importance.