Abstract. Monoclonal antibodies reactive with the tissue form of type VI collagen were used to isolate the type VI collagen polypeptides from cultured fibroblasts and muscle cells. Two [35S]methionine-labeled polypeptides of 260 and 140 kD were found intracellularly, in the medium, and in the extracellular matrix of metabolically labeled cells. These polypeptides were disulfide cross-linked into very large complexes. The 260-and 140-kD polypeptides were intimately associated and could not be separated from each other by reduction without denaturation. In the absence of ascorbic acid, both polypeptides accumulated inside the cell, and their amounts in the medium and in the matrix were decreased. These results suggest that both the 260-and the 140-kD polypeptides are integral parts of the type VI collagen molecule. Examination of type VI collagen isolated from the intracellular pool by electron microscopy after rotary shadowing revealed structures corresponding to different stages of assembly of type VI collagen. Based on these images, a sequence for the intracellular assembly of type VI collagen could be discerned. Type VI collagen monomers are ~ 125 nm long and are composed of two globules separated by a thin strand. The monomers assemble into dimers and tetramers by lateral association. Only tetramers were present in culture media, whereas both tetramers and multimers were found in extracellular matrix extracts. The multimers appeared to have assembled from tetramers by end-to-end association into filaments that had prominent knobs and a periodicity of-110 nm. These results show that, unlike other collagens, type VI collagen is assembled into tetramers before it is secreted from the cells, and they also suggest an extracellular aggregation mechanism that appears to be unique to this collagen.T YPE VI collagen is widely distributed throughout connective tissues (l 3, 24). It has been isolated from aorta and placenta in the form of high molecular weight, pepsin-resistant fragments composed of 50-80-kD polypeptides (3,8,9,19,13,14,16,19,21). It has also been isolated from these tissues by guanidine hydrochloride extraction as a protein composed of polypeptides ranging from 140 kD (10, 15) up to 190 kD (21). One outstanding feature of both the pepsin fragments and the larger forms of type VI collagen is the extensive interchain disulfide cross-linking. It appears from chemical and morphological analyses of type VI collagen that it contains relatively short triple-helical domains, contributing one-third or less to the total mass of the protein (7,10,13,15,19).The molecular architecture (2, 7, 13, 15), the large amount of globular structure, and the extensive disulfide cross-linking in type VI collagen suggest that it is unique among collagens, but little is known about its biosynthesis, processing, and biological function. Type VI collagen is synthesized by fibroblasts and muscle cells in culture and is deposited into their extracellular matrix (l l, 13, 22, 24). The type VI collagen immunoreactivity in cell cultur...