The collagenous components of Ascaris suum intestinal basement membrane were isolated by extraction with 0.1 M Tris-HC1, 0.5 M NaC1, 0.5% 2-mercaptoethanol, pH 8.3, and Sephacryl S-300 gel filtration. Rotary-shadowing electron microscopy showed that the collagenous components occur as monomers and dimers with mean contour lengths of 469 +/- 21 and 918 +/- 24 nm, respectively. The molecules each contain a globular domain, with that of the dimer being slightly larger than that of the monomer. Electrophoresis in sodium dodecyl sulfate-polyacrylamide gels under reducing conditions revealed two polypeptides of Mr = 185,000 and 179,000. A similarity to type IV collagen was indicated by a glycine content of less than 33 mol % and the presence of fucose, mannose, and glucosamine residues. Treatment of the collagen with pepsin resulted in loss of the globular domains but retention of 90% of the length of fibrous collagen segments. Collagenase, however, removed the fibrous regions but left the globular moieties intact. These results extend the previously proposed model (Hung, C.-H., Noelken, M. E., and Hudson, B. G. (1981) J. Biol. Chem. 256, 3822-3826) in which the collagenous domain consists of two monomer-sized triple-helical subunits joined end-to-end by disulfide bonds, with the constituent chains of each subunit being cross-linked by disulfide bonds.