Platelet factor IV (PF4), also called chemokine ligand 4 (CXCL4) is a heparin-binding protein mainly expressed by platelets and megakaryocytes. PF4 has both procoagulant and anticoagulant activity. By heparin binding it inhibits heparin-dependent thrombin activation. In contrast, it also inhibits factor XII of the contact activation pathway and by generation of activated protein C by thrombomodulin binding. PF4 is a 7.8 kDa protein containing 70 amino acids which is secreted from α-granules by activation in a tetrameric form. Ionic strength and pH play an important role in defining the state of the protein complex. Activation takes place during platelet aggregation and its release promotes blood coagulation. These activities indicate a role in wound repair and inflammation. Furthermore, PF4 has been reported to be involved in numerous processes, including hematopoiesis and angiogenesis inhibition, platelet coagulation interference, and host inflammatory response. PF4 is a non-ELR-CXC chemokine and binds to the CXCR3B receptor found on endothelial cells and activated T-lymphocytes. Alternative sources for PF4 are smooth muscle cells, microglia, macrophages and T-cells. Besides the mentioned processes PF4 is also involved in apoptosis, cell differentiation, survival, and differentiation. Since not all cells involved in these processes express the CXCR3B receptor an alternative should exist. Indeed the molecules also exert their actions via binding to, the negatively charged, glycosaminoglycans (GAG; predominantly chondroitin- and heparan sulfates). PF4 capability to inhibit angiogenesis and to promote innate immune responses made it a widely investigated target in tumor intervention and inflammation. Another main clinical topic is heparin-induced thrombocytopenia (HIT). In these patients the PF4-heparin complex acts as a immunogen leading to PF4/heparin antibodies leading to platelet activation and aggregation, thrombocytopenia and thrombosis.Bacterial infections and increasing antibiotic resistance are still an emerging problem. Therefore new ways to eradicate infections are explored. Antibacterial peptides could aid in this strategy. The last 13 amino acids of the carboxyl terminus of PF4 have been found to be antibacterial. The Platelet factor IV peptide has the following sequence: ALYKKLLKKLLKSAKKLG which is an optimized antibactericidal derivative of the last 13 amino acids of the C-terminal end of the human protein.