NK4 is a variant form of HGF/SF, comprising the N-terminal and subsequent four kringle domains of mature HGF/SF. HGF/SF is a multifunctional cytokine that enhances the metastatic behaviour of tumour cells in vitro by stimulation of the c-met receptor tyrosine kinase and has been implicated in the development of tumour metastasis in vivo. The aims of this study were to further investigate the potential antagonistic effects of the recently described variant form of HGF/SF, NK4, on HGF/SF activity in breast cancer cells. All cell lines used expressed both the HGF/SF receptor gene and protein as shown by RT-PCR and Western blotting. NK4 inhibited HGF/SF-induced tumour cell invasion through an artificial basement membrane. Tumour cell motility and scattering induced by HGF/SF were also dramatically reduced by the inclusion of NK4. Immunoprecipitation studies revealed that NK4 inhibited the phosphorylation of the c-met receptor in response to HGF/SF. Treatment of these cells with NK4 alone did not have any significant effects on their metastatic behaviour. From this data we conclude that NK4 demonstrates significant antagonistic properties towards HGF/SF, inhibiting HGF/SF-stimulated breast tumour cell invasion, motility and migration. NK4 may therefore be of potential benefit in the development of anti-metastasis therapies.