Effect of RhoA inactivation by C3 exoenzyme on fibrin clot retraction. Washed platelets were incubated for 4 hours at 37°C with vehicle buffer (“No C3”) or 400 μg/mL of C3. Alternatively, platelets were incubated for 3 hours, 50 minutes without additive, and then with DMSO or 10 μmol/L cytochalasin D (Cyto D) for 10 minutes. Then platelets were added to citrated plasma in siliconized glass tubes, and clotting was initiated with 8 U/mL of thrombin and 2 mmol/L CaCl₂. After 90 minutes at 37°C, clot retraction was quantitated as described in Materials and Methods. Data represent the means of three experiments. Error bars have been omitted for clarity.