Effects of platelet agonists on the exposure of platelet factor XI in normal and factor XI-deficient donors. Four normal donors and four patients with plasma factor XI deficiency were tested for the exposure of platelet factor XI on unactivated platelets and on platelets activated by ADP, thrombin, collagen, or TRP as described in the Materials and Methods. The percentage of exposure of platelet factor XI was calculated as described in the text and in the legends to Figs 1 and 3. The total exposure (100%) of platelet factor XI was defined as the net mean fluorescence intensity of normal platelets activated with 10 μmol/L TRP. (A) depicts the mean percentage of platelet factor XI exposure in four normal platelets, whereas (B) depicts results obtained with platelets from four patients with plasma factor XI deficiency.