Collagen plus thrombin-induced platelet prothrombinase activity. Prothrombinase activity in activated platelets was determined as described previously.10 Briefly, 280 μL of gel-filtered platelets (7 × 10⁷ platelets per mL), 7.5 μL of 100 mmol/L Ca²⁺, and 2.5 μL of Ca-free Tyrode-HEPES elution buffer was activated with (+) or without (−) stirring by addition of 5 μL each of “Horm” collagen (10 μg/mL, final concentration [f.c.]) and thrombin (0.1 U/mL, f.c.). After 10 minutes of incubation at 37°C, 50 μL each of 27 μmol/L prothrombin, 20 nmol/L factor Va, and 20 nmol/L factor Xa was added, and prothrombinase activity measured as the amount of thrombin generated in 1 minute, as described previously.10 Results are shown for MS, the propositus (P), her father (F ), mother (M), and son (S) as percent of mean normal control (C) values, which were 1,831 pmol/10⁸ platelets/min for stirred platelets and 1,450 for unstirred platelets (n = 8). The error bars indicate the mean − 2SD.