Decreased thrombin generation in platelets lacking the GPIb-V-IX complex. Calibrated automated thrombin (CAT) experiments were performed in citrated platelet-rich plasma (PRP; final platelet count 1.3 × 10⁸ cells/mL). The reaction was triggered by addition of tissue factor (TF; 0.5pM) or collagen-related peptide (CRP; 10 μg/mL) in the presence of calcium and the fluorogenic substrate Z-GGR-AMC. Thrombin activity was determined from the accumulation of the fluorescent product and was calculated relative to a thrombin calibrator. (A-B) Representative thrombin generation curves in PRP from GPIbβ^−/− () and GPIbβ^+/+ mice (■). (C) Endogenous thrombin potential (ETP) values, representing the total amount of thrombin generated. Peak values represent the maximal thrombin concentration attained. The results are presented as the mean ± SEM in 7 (TF) or 4 (CRP) independent experiments. **P < .01, *P < .05 versus wild-type (WT) mice.