Type IV collagen was solubilized from a tumor basement membrane either by acid extraction or by limited digestion with pepsin. The two forms were similar in composition and the size of the constituent chains but differed when examined by electron microscopy and in the fragment pattern produced by bacterial collagenase. The acid-soluble form showed after rotary shadowing strands mainly of a length of 320 nm which terminated in a globule, or two strands connected by a similar globule. The globule was identified as a non-collagenous domain (NCl) which under dissociating conditions could be separated into two peptides showing a monomer-dimer relationship. Higher aggregates of NCl were visualized under non-dissociating conditions. Some of the acid-extracted molecules have retained the previously described 7-S collagen domain. The pepsin-solubilized form lacked domain NCl and consisted mainly of of four triple-helical strands (length 356nm) joined together at the 7-S domain (length 30 nm.). Common to both forms of type IV collagen was a small collagenase and non-collagenous elements and located between the 7-S domain and the major triple helix. These data indicate that the collagenous matrix of basement membranes consists of a regular network of type IV collagen molecules which is generated by two different interacting sites located at opposite ends of each molecules. The 7-S collagen domain connects four molecules while the NCl domain connects two molecules. The maximal distance between identical cross-linking sites (7-S or NCl) was estimated to about 800 nm comprising the length of two molecules.