The extraction, isolation and partial characterization of heparin-like anticoagulants from the 2 spp. of clams, Mactra spissula and Artica islandica, are described. These purified polysaccharides, designated mactin-A and mactin-B, are related to but not identical with mammalian heparin, and give an anticoagulant response in rabbits and cats which is greater than that obtained with standard Na heparin. Acute and chronic toxicity studies in a variety of animals reveal that the therapeutic index of both mactin-A and mactin-B is comparable to that of heparin.