Fibrin activates FXII and elicits FXIIa-antithrombin (AT) complex formation in blood. Fibrin clots were produced at the bottom of tubes. Three different thrombin (thr)/ fibrinogen (fib) ratios generated clots with low (L), intermediate (I), and high (H) density. (A) The clots or native fibrinogen were incubated with 1 μg of purified FXII, and the enzymatic activity of FXIIa and thrombin was measured using the chromogenic substrates S-2302 (FXIIa) and S-2238 (thrombin). Data are presented as the means ± standard error of the mean (SEM) (n = 3). (B-C) The clots, native fibrinogen, and thrombin were incubated with lepirudin-anticoagulated platelet-poor plasma (PPP), platelet-rich plasma (PRP), and whole blood. The generation of FXIIa-AT (A) and FXIa-AT (B) by the clots in all blood systems was measured by ELISA. Data are presented as the mean ± SEM (n = 5), and differences compared with the base level are shown (Kruskall-Wallis test with Dunn’s multiple comparison).