Platelet factor 4 (PF4) is a small cytokine belonging to the CXC chemokine family that is also known as chemokine (C-X-C motif) ligand 4 (CXCL4). This chemokine is released from alpha-granules of activated platelets during platelet aggregation, and promotes blood coagulation by moderating the effects of heparin-like molecules. Due to these roles, it is predicted to play a role in wound repair and inflammation. It was reported that PF4 can inhibits U937 cells proliferation. To test the effect of U937 on cell apoptosis, U937 cells were seeded into 96-well plates at a density of 5,000 cells/well with 5% serum standard 1640 including various concentrations of recombinant Human PF4. After incubated for 48 h, cells were observed by inverted microscope and cell proliferation was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10 µL of CCK-8 solution was added to each well of the plate, then the absorbance at 450 nm was measured using a microplate reader after incubating the plate for 4 hours at 37℃. Proliferation of U937 cells after incubation with PF4 for 48 h observed by inverted microscope was shown in Figure 1. Cell viability was assessed by CCK-8 assay after incubation with recombinant Human PF4 for 48 h. The result was shown in Figure 2. It was obvious that PF4 significantly inhibit cell viability of U937 cells. The ED50 is 2.2-3.1μg/mL.