Twenty-two algal species were evaluated for their potential anticoagulant activities. Hot water extracts from selected species, Codium fragile and Sargassum horneri showed high activated partial thromboplastin time (APTT). Ultraflo extract of C fragile and S. horneri exhibited the most potent anticoagulant activity. Furthermore, in both algal species, active compounds were mainly concentrated in > 30 kDa faction. The crude polysaccharide fraction (> 30 kDa; CpoF) of C fragile composed of similar to 80% carbohydrate and similar to 19% of protein; the crude polysaccharide fraction (> 30 kDa; CpoF) of S. horneri was composed of 97% of carbohydrate and similar to 2% of protein. Therefore, most probably the active compound, or compounds of the algal species were related to high molecular weight polysaccharide, or a complex form with carbohydrate and protein (proteoglycan). (c) 2006 Elsevier Ltd. All rights reserved.