The brown seaweed Spatoglossum schröederi (Dictyotaceae) contains three main fucans (fucans A, B and C) with different mobility in electrophoresis. The fucan with highest mobility (fucan C) was precipitated with 2.0 volumes of acetone, purified using a combination of ion exchange chromatography and electrophoresis. It showed an MW of 24 kDa determined by HPLC and Sephadex G-75 chromatography and migrates as a single band in three distinct electrophoretic systems. This fucan contains fucose, xylose, galactose and sulfate in a molar ratio 1 : 0.6 : 2:2.3. The fucan has neither anticoagulant (from 10 to 100 microg) nor hemorrhagic activities (100 microg/mL). In addition, fucan C is neither cytotoxic nor cytostatic. However, fucan C (100 microg/mL) stimulated the synthesis of an antithrombotic heparan sulfate from endothelial cells of rabbit aorta. The results suggest that fucan C might be used as an antithrombotic therapeutic compound.