Type IV collagen incubated at elevated temperatures in physiologic buffers self-associates (a) via its carboxy-terminal (NC1) domain, (b) via its aminoterminal (7S) domain, and (c) laterally; and it forms a network. When examined with the technique of rotary shadowing, isolated domain NC1 was found to bind along the length of type IV collagen to four distinct sites located at intervals of ~100 nm each. The same 100-nm distance was observed in domain NC1 of intact type IV collagen bound along the length of the collagen molecules during initial steps of network formation and in complete networks. The presence of anti-NC1 Fab fragments in type IV collagen solutions inhibited lateral association and network formation in rotary shadow images. During the process of selfassociation type IV collagen develops turbidity; addition of isolated domain NC1 inhibited the development of turbidity in a concentration-dependent manner. These findings indicate that domain NC1 of type IV collagen plays an important role in the process of selfassociation and suggest that alterations in the structure of NC1 may be partially responsible for impaired functions of basement membranes in certain pathological conditions.