"Beaded filaments" have been found in fibroblast cultures prepared from chicken embryo leg tendons and cornea and from the dermis of human skin. With negative staining, they appear as single, unbranched, flexible strands approximately 3 nm in width and up to at least 2 microns in length. Pairs of "beads" are distributed on the filament at regular intervals of 110 nm. The beaded filaments appear to be resistant to the action of trypsin and bacterial collagenase. The filaments also occur in bundles with beads laterally aligned to form long-spacing-type fibrils, which appear to be identical with many fibrous-long-spacing-type fibrils described, but not identified, by others in a variety of normal and pathological tissues. The long-spacing fibrils are numerous at several sites of active collagen fibrillogenesis. Comparison of the beaded filament structure with molecular models for various collagens described in the literature suggests that they are a filamentous form of type VI collagen.