The subunits of the collagen IV hexameric, noncollagenous NC1 domain obtained from bovine aorta, glomerular basement membrane, alveolar basement membrane and placental basement membrane are predominantly dimers. A large fraction of the dimers had been thought to be linked by nondisulfide bonds because they were resistant to cleavage by 5% (v/v) 2-mercaptoethanol, 2% (w/v) SDS, at 100 degrees C. However, if an unusually high concentration of 2-mercaptoethanol, e.g., 40% (v/v), is used, complete conversion of dimers into monomers is achieved, indicating the lack of intersubunit nondisulfide cross-links. Electrophoresis patterns indicate that some of the intermolecular disulfide bonds of the dimers are more resistant to reduction in aqueous SDS than are some of the intramolecular disulfide bonds.