For the comparison of tissue-type plasminogen activator (t-PA) measurements performed in different laboratories, standardization is required. The large-scale production of t-PA was achieved first from high-producing cell lines and more by recombinant DNA technology. Due to this increased interest in t-PA, the need for more rapid high-yielding purification schemes together with easy and reliable assay procedures became pertinent. Assay procedures for plasminogen activators can be based on their enzymatic or immunological properties. The activity assays are arranged according to the substrate of the activator plasminogen and synthetic peptides. By their nature, immunological assays measure the presence of the particular antigen. Enzyme immunoassays have been developed based on the principle of immunohisto-chemistry. Activity assays based on ultimate detection of plasmin activity have in general a ten-to hundred-fold lower detection limit than most immunological procedures. The most serious disadvantage of radioimmunological assays is the instability of the radiolabeled reagents.