The antigen of monoclonal antibodies which had labeled the hexagonal lattice of Descemet's membranein a specific manner was shown to be the α1 chain of type VIII collagen by immunoblotting followed by amino acid sequence analysis. With this antibody, the localization of α1 (VIII) in various tissues was studied by several immunocytochemical methods. Under light microscopy, the α1 (VIII) was found in a fine fibrillar form in various capsular tissues such as capsules of the liver, kidneys, adrenals, lungs and so on. It was also present in dense connective tissues such as the Achilles tendon, and periodontal and perivertebral ligaments. When some dense connective tissues which had been negative to the label including the intima of aorta, perimysium and Glisson's sheath of the liver, were subjected to pepsin digestion, epitopes were revealed which showed a specific immunofluorescence pattern. In many locations the pattern of localization coincided with that of elastic fiber components, and full or partial colocalization with tropoelastin or costaining with resorcin-fuchsin staining was observed. In immunoelectron microscopy, the antigen (α1 (VIII)) was localized on the surface of, but not inside, elastic fibers. However, some tissues which are rich in elastic fibers or microfibrils remained unlabeled. These included elastic fibers of the aortic media and ligamentum nuchae as well as ciliary zonules. Therefore it is suggested that α1 (VIII) is a collagen associated with microfibrils of some elastic fiber systems, but is not an intrinsic component of either elastic fibers or of microfibrils.