Platelet polyP is a much weaker activator of FXII than kaolin. FXIIa generation in citrated human platelet-free plasma was measured using an FXIIa-sensitive fluorogenic substrate in the presence of different concentrations of platelet polyP or kaolin (n = 5 normal and 3 FXII-deficient donors). Substrate fluorescence was measured for 30 minutes, and the maximum slope (from 5 neighboring data points) was calculated and is presented as mean ± standard deviation (SD).