Protamine prolongs in vivo bleeding time and clotting time of normal pooled plasma. BALB/c mice were injected with either phosphate-buffered saline or 5 mg/kg protamine. Tail-bleeding time (A) or, in separate mice, plasma endogenous thrombin potential (B) was measured, as described in “Methods.” In addition, the effect of protamine on clotting times of human normal pooled plasma was characterized. Increasing concentrations (10-120 μg/mL) of protamine were titrated into normal pooled plasma, and the effect on plasma APTT (C) and plasma PT (D) was determined. Results described represent the mean ± SEM (*P < .05, ***P < .001, respectively; unpaired 2-tailed t tests performed).