We used monospecific antibodies on semithin frozen sections to identify and localize the major tissue constituents of the nonglaucomatous human trabecular meshwork. The trabecular beams (sheets and cords) consist of a basement membrane (subendothelial extracellular matrix) surrounding an interstitial central core of connective tissue (substantia propria). The basement membrane contains collagen types III, IV, and V, the glycoproteins laminin and fibronectin, and the basement membrane-associated heparan sulfate proteoglycan. The trabecular basement membrane is unlike most subendothelial basement membranes because it contains collagen type III and a relatively disorganized structure. The central core contains collagen types I and III, and elastin. The closely linked juxtacanalicular meshwork contains collagen type III, but no collagen type I or elastin. The connective tissue composition of the trabecular meshwork appears similar to other highly compliant and resilient tissues, such as lung, blood vessels, and conjunctiva.