Comparison of the exposure of platelet factor XI, GPIIb/IIIa, and P-selectin on platelets. One hundred percent binding of anti-factor XI antibody, FITC-PAC1, or FITC-S12 to platelets was defined as the mean fluorescence intensity of platelets activated by 10 μmol/L thrombin receptor peptide (SFLLRN amide) affinity-purified factor XI antibody and incubated with PAC1 antibody as described in the Materials and Methods. The results shown are the means (±SEM) of data obtained with platelets from four normal donors probed for platelet factor XI (•), GPIIb/IIIa (□), or P-selectin (○) .