Earthworm extracts are known for anti-inflammatory, analgesic. antipyretic, and anticancer effects but can also influence blood circulation. It was previously shown that an earthworm, Lumbricus rubelius. contained a water-extractable anticoagulant which was a heat- and acid-stable molecule with hydrophilic property. In order to uncover the biochemical nature of this molecule, the anticoagulant was processed with various hydrolases such as trypsin, DNase, RNase. and lysozome. When the digested samples were analyzed with an in vitro coagulation test measuring activated partial thromboplastin time (APTT) and agarose gel electrophoresis, the anticoagulant proved to be a relatively homogeneous DNA fragment with relative molecular size around 72 base pairs. Interestingly, the activity was further stimulated with a trypsin digestion. RNA. on the other hand, did not prolong the APTT. It was also demonstrated that the DNA accelerated the antithrombin III (AT-III) inhibition of thrombin from $IC_{50}$ of 0.34 to 0.16 unit determined with S-2238 as a substrate, whereas heparin, a popular anticoagulant. shifted the value to 0.05. Therefore, it is suggested that the DNA could be considered as an alternative antithrombotic agent to heparin, which would exhibits bleeding side effects.