Hepatocyte Growth Factor/Scatter Factor (HGF/SF) is a heterodimeric molecule that plays a key role in the regulation of migration, invasion and angiogenesis in cancer, via activation of its receptor, c-met. HGF/SF is composed of an alpha-chain, containing the N-terminal hairpin domain and 4 kringle domains, plus the serine protease-like beta-chain. We have examined here the properties of NK4, an HGF variant containing the N-terminal hairpin plus the 4 kringle domains, on tumour cell proliferation, dissociation and invasion using human colorectal cancer cells (HT115). The expression of HGF/SF and its receptor was also examined by RT-PCR and Western blotting. Analysis revealed NK4 to be an HGF/SF antagonist that, at a wide range of concentrations, did not exert any biological effects of its own. HT115 cells were shown to express the HGF/SF receptor mRNA and protein. HGF/SF-induced receptor tyrosine phosphorylation was suppressed, in a dose-dependent manner, upon addition of NK4, whereas the addition of NK4 alone caused no phosphorylation. Tumour cell motility was induced by HGF and inhibited by NK4. Furthermore, HGF/SF induced the invasion of cells through Matrigel basement membrane components, and again this induced invasion was suppressed by NK4. Our results show that the ability of HGF/SF to stimulate tumour cell motility and invasion, properties required for metastatic spread, can be inhibited by NK4. Thus, NK4 may have an important role in the control of cancer metastasis.