The plasminogen-plasmin system is involved in fibrinolysis and probably in several other processes where extracellular proteolysis takes place. At least two types of plasminogen activators occur in the human body: tissue-type plasminogen activator (t-PA), originally identified in tissue extracts, and urokinase-type plasminogen activator originally found in urine. The structure of t-PA will be compared with that of other proteases in order to obtain additional information about the functions of certain structures. The primary function of t-PA and other plasminogen activators is to hydrolyse one specific Arg-Val peptide bond in the plasminogen molecule. The amino acid sequence of t-PA has been deduced from the cDNA sequence of human melanoma t-PA and then confirmed by peptide analysis. t-PA purified from tissues or culture fluids usually exists for the greater part of two-chain t-PA. Both single-chain and two-chain t-PA have amidolytic activity towards synthetic tripeptide substrates.