Unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) are important clinical anticoagulants. As polynegative molecules they are potential triggers of the contact phase of coagulation. An incubation temperature lower than the physiological 37°C favours intrinsic haemostasis activation by the polynegative molecule SiO2. The efficiency of UFH and LMWH after a plasmatic preincubation at 37 or at 23°C is therefore studied. Samples (150 μl) of unfrozen pooled normal plasma supplemented with 0, 0.01, 0.1, or 1 IU/ml heparin or dalteparin in 5-ml polystyrole tubes were incubated for 10–70 min at 37 or at 23°C. The extrinsic coagulation activity assay (EXCA) was then performed. Preincubation at 37°C of 0.1 IU/ml plasmatic UFH does not result in any thrombin generation in EXCA-1, whereas preincubation at 23°C results in a thrombin generation of about 0.1 IU/ml thrombin. Plasmatic UFH (0.01 IU/ml) at 23°C acts nearly half as efficiently as 0.01 IU/ml plasmatic LMWH. Polynegatively charged niches particularly in the larger UFH molecule might trigger the contact system of haemostasis, especially at 23°C. In contrast, the anticoagulant capacity of LMWH does not change significantly with temperature.