Type IV procollagen was isolated from the culture medium of the teratocarcinoma cell line PYS-2 by affinity chromatography on heparin-Sepharose. Immunological studies showed that type IV procollagen is composed of pro-alpha 1(IV) and pro-alpha 2(IV) chains and contains two potential cross-linking sites which are located in the short triple-helical 7S domain and the globular domain NC1 . The 7S domain was also identified as the heparin binding site. Rotary shadowing visualized type IV procollagen as a single triple-helical rod (length 388 nm) with a globule at one end. Some of the procollagen in the medium, however, had formed aggregates by alignment of 2-4 molecules along their 7S domains. After deposition in the cell matrix, non-reducible cross-links between the 7S domains are formed while the globules of two procollagen molecules connect to each other. The latter may require a slight proteolytic processing of the globular domains NC1 . The shape of type IV procollagen and the initial steps in its assembly are compatible with a recently proposed network of type IV collagen molecules in basement membranes. Since both type IV collagen and laminin bind to heparin, the formation of higher ordered structures by interaction of both proteins with heparan-sulfate proteoglycan may occur in situ.