Platelet factor 4 is a low molecular weight protein contained in the storage granules of platelets and released during aggregation with a variety of aggregating agents. In vitro, it is a potent antiheparin. This property has been used for a rapid, simple purification procedure using affinity chromatography on heparin epsilon-aminocaproic aced Sepharose. Supernatants collected from outdated platelet concentrates, or platelet extracts prepared from washed, outdate platlets themselves, are first precipitated with 50% ammonium sulfate. The supernatant is dialyzed and applied to the affinity column. Contaminating proteins are washed from the column with 0.5 M NaCl in 0.005 M sodium barbital buffer, pH 7.4 and the column is then eluted with a gradient of 0.5 to 3.0 M NaCl in 0.005 M sodium barbital buffer, pH 7.4. When prepared from platelet extracts, a single protein peak with high platelet factor 4 activity is eluted at 0.9 to 1.0 M NaCl. The peak fractions demonstrate a single band on Na dodecyl-SO4-polyacrylamide gel electrophoresis. The molecular weight as determined by Na dodecyl-SO4 gel electrophoresis was 11,600 +/- 330, and was 40,000 by gel filtration.