The capsular polysaccharide produced by the thermophilic blue green alga/cyanobacterium Mastigocladus laminosus was tested for its cytotoxic activity against the A431 human epidermoid carcinoma cell line. This polysaccharide inhibited the proliferation of A431 cells in a dose-dependent manner with an IC (50) value of 50 microg mL (-1). In addition, this polysaccharide strongly inhibited A431 cell migration and invasion. Preliminary experiments showing that secretion of metalloproteinases MMP2 and MMP9 by A431 tumour cells was inhibited by this polysaccharide suggest that this mechanism of action could play a role in its anti-migration and anti-invasive properties. Acid hydrolysis of the polysaccharide produced specific oligosaccharides which conserved - at similar concentrations - their cytotoxic, anti-migration and anti-invasion properties; in this case, the mechanism of action was nevertheless uncorrelated to the decrease of metalloproteinase expression.