In vitro clotting times in the Amelung instrument are more FXIIa dependent in platelet-free plasma containing phospholipids than in whole blood. Clotting times for human whole blood with resting or activated platelets (Trap6, 30 µM) and platelet-free plasma with added phospholipids (2.5 µM) to substitute for negatively charged platelet membranes were measured with or without FXIIa inhibitor (CTI, 50 µg/mL) or phosphatase (10 U/mL, purchased in accordance with instructions from Prof Renné). Data are presented as mean ± SD (whole blood: n = 10, platelet-free plasma: n = 3), and statistical testing was performed with repeated-measures ANOVA and Dunnett’s post hoc test for whole blood experiments.