Armatan, an acid sulphated polysaccharide isolated from a red marine seaweed Asparagopsis armata (Harv.), increases the coagulation time of the rat plasma in vivo assays. This property presents, with administration, time and dose, some variations. Among the three ways studied, subcutaneous injection gives more interesting results than the others. At high concentrations (10, 20 and 50 mg/kg), intravenous administration induces plasma and organic (liver-spleen) diseases.