Clotting of human plasma by human a-thrombin was prolonged in the presence of haemoglobin as was human and bovine fibrinogen. Specifically, the clot time doubled for human plasma, human fibrinogen and bovine fibrinogen at 483, 233, and 116 μM haemoglobin, respectively. Fibrinopeptide A release was not inhibited at concentrations in ~16000 molar excess compared with α-thrombin. Turbidometric analysis of fibrin polymerization showed a lengthening of the lag phase as well as the fibrin assembly process in the presence of haemoglobin. These findings suggested that neither fibrinogen recognition nor catalytic efficiency of thrombin was affected, implying that haemoglobin interferes with fibrin polymerization. Since human blood contains sufficient haemoglobin in erythrocytes to generate concentrations of up to 2.3 mM upon cell lysis, and haemoglobin concentrations of 0.16–0.48 mM caused 1.25 to two times longer clotting times in fresh human plasma, respectively, haemoglobin may act to modulate clot formation under conditions of haemolysis.