An electron microscopy study on the distribution of elastin in the trabecular meshwork of normal human eyes and in that of eyes with primary open angle glaucoma (POAG) was done using a protein-A gold immunohistochemical method with antiserum to either alpha-elastin or tropoelastin. Four types of elastic fibers were found to have elastin: (1) elastic-like fibers without the sheath, (2) fibers surrounded with a sheath of periodic structure, (3) fibers surrounded with fine granular-like material, and (4) connecting fibrils. No individual differences were observed in the labeling for these four types of elastic fibers. Antigenic sites against elastin of the elastic fibers were observed mainly in the low electron density amorphous elements. In the subendothelial layer of Schlemm's canal of the eyes with POAG, a marked increase of elastin was noted within the area containing fine fibrillar-like material. This phenomenon did not occur in age-matched normal eyes. The results suggest that elastin plays an important role in the development of POAG.