The interaction between polyoxometalate (POM) of Keggin class (K5SiW11O39Co) and human basic fibroblast growth factor (bFGF) was studied by fluorescence and CD spectra. The results indicate that the binding of POM to bFGF causes conformational changes of bFGF. The cell proliferation assay showed that in lower molar ratios, POM stimulates mitogenic activity of bFGF; in higher molar ratios, POM inhibits the mitogenic activity of bFGF.