Plasminogen is the zymogen of the proteolytic enzyme plasmin (EC 3.4.4.14), the enzyme responsible for the dissolution of fibrin clots in blood. In a continuing effort to resolve problems of homogeneity, yield, stability, reproducibility, solubility at physiological pH, and isolation of the native form of the zymogen, various methods have evolved over the three decades following the original euglobulin precipitation procedure for concentrating plasminogen from plasma. These efforts culminated in combining gel filtration and ion-exchange chromatography techniques as the procedures of choice. The development of affinity chromatography in recent years' provided an attractive alternative to conventional multistep purification procedures as a gentle and effective technique for the purification of plasminogen. The first applications of affinity chromatography for the purification of plasminogen used an L-Lysine substituted agarose to obtain the zymogen from human plasma. Lysine was used as the ligand on the basis of structural similarities between the covalently linked lysine (presumably through the, α-amino group), and ∈-aminocaproic acid. Lysine-agarose was shown to specifically bind plasminogen from blood plasma, and the retarded zymogen was eluted with ∈-aminocaproic acid. This single step purification of plasminogen provided a preparation comparable in purity and homogeneity to those obtained by conventional multistep procedures, and was the first reported preparation of a proenzyme by affinity chromatography.