Investigation of inhibitory effect of two single-stranded DNA thrombin-inhibiting aptamers (15TBA and 31TBA) on fibrin polymerization in fibrinogen solutions and comparison of anticoagulant properties of these aptamers by a new global coagulation test of thrombodynamics. Measurement of aptamers' functional stability in human plasma and blood in vitro in order to investigate the involvement of 3'-exonuclease in fast decrease of aptamers' functional activity in vivo. Thrombin inhibition activity was measured in a buffer system in vitro as effects of aptamers on fibrin polymerization. Anticoagulant activity was investigated by measuring the spatial clot growth rate in the presence of aptamers. The stability of aptamers during incubation in human plasma was investigated in vitro by measuring activated partial thromboplastin time. Both aptamers dose-dependently inhibit fibrin polymerization in a buffer solution (IC50=10 nm for 15TBA and 3 nm for 31TBA) and are effective anticoagulants in human plasma (IC50 for spatial clot growth rate decreasing are 9.5 μmol/l and 4.0 μmol/l for 15TBA and 31TBA, correspondingly). Both aptamers remain stable in plasma or whole blood in vitro for at least 4 h. It was shown that 31TBA was 2-3 times more effective than 15TBA. Both aptamers were stable in human plasma and whole blood in vitro. So, the 3'-exonuclease could not be the reason for fast decrease of aptamers' functional activity in vivo. The main role in the removal of oligonucleotides from the circulation is played obviously by the liver.