Clot retraction kinetics. Platelets (2 × 10⁸ mL⁻¹ final concentration) were preincubated for 10 minutes at 37°C with 300 nmol/L final concentration of the indicated fibrinogen before adding human α-thrombin to 0.5 U/mL final concentration. The length and width of the clot were measured with a ruler every 2 minutes and used to calculate clot area, which was then used to calculate % retraction. (A) Average % retraction, expressed as mean ± standard error from four experiments, is plotted versus time as a bar graph for the various recombinant fibrinogens, which are indicated in the legend. (▧), Normal recombinant; (□), Aα D97E fibrinogen; (▤), Aα D574E fibrinogen; (▪), Aα DE²/γ 407 fibrinogen. (B) Curves generated by plotting average % retraction versus time for normal recombinant and Aα DE²/γ407. (▪), Normal recombinant; (•), Aα DE²/γ 407 fibrinogen.