Type IV collagen is known as a major component of basement membranes [11. It contains two different polypeptide chains (review [2]) which differ from the OL chains of the interstitial collagens type I, II and III in amino acid composition [I], a larger M,-value [3-71 and in frequent interruptions of the triple helix by non-helical regions [8]. These interruptions are responsible for the high sensitivity of basement membrane collagen to proteolytic attack. In] 9, 10] unusual collagen structures were isolated from basement membranes of a tumor matrix, placenta, lens capsule and kidney after limited proteolytic digestion. These structures named 7 S collagens appeared in a long and a short form, with app. Mr 360 000 and 200 000, respectively. The data suggested that 7 S collagen is composed of several triple helical segments with a high denaturation temperature of 7O” C, due to multiple disulfide bonds between the chains [lo]. Because of the high stability of the triple helix, 7 S collagen is resistant to intensive treatment with pepsin or bacterial collagenase under conditions where other triple helical structures would be digested [9]. In previous studies it could not be decided whether 7 S collagen represent a special domain of type IV collagen involved in intermolecular crosslinking or a fragment of a new, unusual type ef collagen. By using the rotary shadowing technique we have now compared the size and dimensions of 7 S collagen and of polymeric forms of type IV collagen from a mouse tumor and human placenta. The data demonstrated that 7 S collagen is a domain of type IV collagen in which 4 triple helical molecules are held