BP180 is a glycoprotein constituent of the epidermal anchoring complex and a major antigenic target of autoantibodies associated with bullous pemphigoid, a blistering skin disease. The C-terminal extracellular domain of BP180 contains 15 domains composed of Gly-X-Y tandem repeats, which are predicted to form collagen-like triple helices. To facilitate the structural analysis of this protein, the extracellular region of human BP180 was expressed as a secreted protein (sec180e) in transiently transfected COS-1 cells. Gel filtration and sedimentation analyses demonstrated that sec180e exists in two forms:  a globular monomeric form and a high-molecular mass multimeric form with an elongated conformation. Pulse−chase and cross-linking experiments established that the sec180e complex is a stable homotrimeric structure which assembles prior to secretion from the cell. On the basis of its calculated molecular mass, the oligomeric state of the sec180e complex is 3.25. With a Stokes radius of 13.6 nm, a sedimentation coefficent of 6.5 S, and a frictional ratio of 3.01, the sec180e protein appears to be highly extended (length to width ratio is between 52 and 60), yet is more flexible than a rigid rod. BP180 isolated from human epidermis was also shown to exist in a high-molecular mass complex which, like sec180e and other collagenous proteins, is SDS-stable but heat-labile. These findings strongly suggest that the BP180 ectodomain exists as an elongate, flexible homotrimer. This trimerization is likely to result from the formation of stable collagen triple-helical and coiled-coil type structures and does not depend upon the presence of the cytoplasmic or transmembrane domains of this protein.