Flow cytometric assay for the detection of platelet factor XI on the surface of normal platelets. The experiment was performed as described in the Materials and Methods. The Y axis displays the number of platelets at any specific fluorescence intensity noted on the X axis as a log scale. The C gate was set at the edge of the background of platelet fluorescence intensity without adding any primary antibody or preimmune goat IgG. A representative result is shown, in which affinity-purified anti-factor XI antibody was bound to unactivated platelets (A) and to platelets activated by 10 μmol/L thrombin receptor peptide (B). Results in (C) depict preimmune goat IgG nonspecifically bound to unactivated and (D) to activated platelets. The mean fluorescence intensity in (A) minus that in (C) or (B) minus (D), respectively, represent the net mean fluorescence intensity due to anti-factor XI antibody binding to platelet factor XI.