In dolphins, blood pooling and acidosis from lack of oxygenation with prolonged underwater stay are not associated with intravascular clotting as it would be in terrestrial mammals, while shed blood clots promptly, and intravascular clots form after death. This intriguing physiologic adaptation prompted further investigation of the coagulation system in the dolphin. We studied the plasma from 17 dolphins (Tursiops truncatus) for coagulation factors and from 12 of the 17 for natural anticoagulants and fibrinolytic products using reagents and equipment currently used for humans. We obtained the following results: prothrombin time, 13.3±0.5 s, activated partial hromboplastin time, no clot; fibrinogen, 343±74 mg/dl; functional assays of factors II, 0.4±0.07 U/ml; V, 2±0.5 U/ml; VII, 0.5±0.08 U/ml; VIII, 7.5±1 U/ml; IX, 2.4±0.5 U/ml; X, 0.9±0.1 U/ml; XI, 0.8±0.2 U/ml; and XII, not detected, antithrombin functional assay, 105.7±12.5%; antithrombin antigen, 96±8.3%; protein C activity 125.1±10.8%; C antigen, undetectable; protein S activity 15.6±6.8%; S antigen, undetectable. Fibrin degradation products and D-dimer were not increased above the normal human range. Our study suggests that the contact system in healthy dolphin may play a role in hindering intravascular clotting of stagnant blood. Low levels of functional protein S were not associated with any increased risk for thrombosis.