Abstract Perlecan, a major basement membrane proteoglycan, has a complex modular structure designed for the binding of many cellular and extracellular ligands. Its domain IV, which consists of a tandem of immunoglobulin-like modules (IG2-IG15), is rich in such binding sites, which have been mapped to different modules obtained by recombinant production. Heparin/sulfatide binding was restricted to IG5 and shown to depend on four arginine residues that are close in space in β strands B and E of the C-type IG fold. The nidogen-1 and nidogen-2 isoforms bind to IG3 with high affinity ( K d∼10 nM). This interaction depends on the globular nidogen domain G2 and is crucial for the formation of ternary complexes with laminins. Two loops of IG3 located between β strands B/C and F/G, which are spatially close, make a major contribution to binding. Fibronectin binding was localized to IG4-5 and fibulin-2 binds to IG2 and IG13-15 with different affinities. This implicates a complex cluster of heterotypic interaction sites apparently important for the supramolecular organization of perlecan in tissues.