Histones induce thrombocytopenia. Histones bind to platelets in blood. (A) Fluorescent histones were mixed at the indicated concentrations with blood. Flow cytometric analysis showed that histones bind preferentially to platelets over RBCs in blood (**compared with RBCs; n = 5). (B) Perfusion of Rhodamine 6G–labeled platelets in blood over a histone-coated surface shows platelet binding to histones. Areas covered with BSA only did not bind platelets. Scale bar = 100 μm. (C) Histones induce thrombocytopenia in blood in vitro. Single platelet and RBC counts of blood mixed with indicated concentrations of histones. At concentrations of 250 μg/mL histones induced thrombocytopenia in vitro (***compared with RBCs; n = 3). (D) Microscopic analysis of Rhodamine 6G–labeled platelets in blood before (Unstimulated) and after stimulation with Alexa 488–labeled histones (+Histones). Histones induce and localize to platelet aggregates in blood. Scale bar = 50 μm. Histones induce thrombocytopenia in vivo. (E) Platelet and RBC counts of mice 10 minutes after infusion with indicated sublethal concentrations of histones (***compared with RBCs; n = 3). Histone infusion caused dose-dependent depletion of platelets from circulation. (F) Determination of tail bleeding time. Mice infused with histones (50 mg/kg) but not vehicle showed prolonged bleeding time. Histones associate with platelets in vivo. Immunostaining of lungs dissected 10 minutes after infusion with histones (G-I; 50 mg/kg) or vehicle (J). Lungs were stained for CD41 (platelets), histone H3 (histones), and DNA. Platelets (G) and histones (H) colocalized in mice infused with histones (I) but not in mice infused with vehicle (J). Under these conditions, histone staining of nuclei was below the detection limit. Scale bar = 50 μm. Data presented are representative of ≥ 3 independent experiments. **P < .01, ***P < .001.