The FNIII13 domain of fibronectin and smaller fragments thereof have a tumour cell proliferation inhibitory effect. Compositions are provided comprising fragments of fibronectin having the FNIII 13 domain and fragments thereof. A system comprising cells exposed to fibronectin and caused to proliferate by the presence of tenascin are used as an in vitro method for screening possible anti-tumour agents. Cell-free systems comprising a fibronectin ligand and tenascin are also employed for screening potential anti-tumour or anti-cancer agents. Test compounds are assayed for the ability to disrupt binding of the fibronectin ligand to tenascin. A further cell-free system additionally includes a syndecan molecule.