Carbon monoxide (CO) has been demonstrated to enhance coagulation and attenuate fibrinolysis in vitro and in vivo. Hemostasis is affected by CO interactions with key heme-modulated molecules. We wished to determine whether freezing would affect CO-mediated changes in coagulation/fibrinolysis in plasma in anticipation of collecting samples both within our institution and from collaborating centers. Plasma was exposed to CO by addition of 0-100 μmol/l tricarbonyldichlororuthenium (II) dimer, with a portion of plasma immediately frozen at -80 °C. Unfrozen plasma was subjected to thrombelastographic analysis following tissue factor activation, with some samples exposed to tissue type plasminogen activator. Frozen plasma was subsequently thawed and similarly analyzed. Freezing did not significantly change CO-mediated enhancement of coagulation or attenuation of fibrinolysis. Hemostatic changes in plasma exposed to CO are not affected by a freeze-thaw cycle, which will permit local batch processing of samples and transport of samples on dry ice from collaborating centers.