Alpha-2-antiplasmin, a major inhibitor of fibrinolysis, is synthesized in the liver and occurs in blood in two molecular forms: a very active plasminogen-binding (PB) form and a less active nonplasminogen-binding (NPB) form. This study investigated the origin and mutual relationship of these two forms in vivo and in vitro. Despite wide variation in plasma concentration of the inhibitor (16% to 138%), the ratio between the two forms in vivo was found to be, in the main, constant among healthy volunteers, heterozygotes for a congenital deficiency of alpha-2-antiplasmin, and patients with a stable liver cirrhosis: PB/NPB = 2.41 ?? 0.34 (SD). Resynthesis after depletion or increased synthesis in the acute-phase reaction showed a specific increase of the PB form of the molecule in blood after discontinuation of L-asparaginase or streptokinase therapy and after myocardial infarction. In vitro studies demonstrated that only the PB form was present after one day in the culture medium of the human cell line Hep G2, while the NPB form appeared after 11 days. Clearance after inhibition of synthesis by L-asparaginase therapy revealed a more rapid decrease in the PB form relative to the NPB form in blood, demonstrated by a change in the PB-NPB ratio from 2.86 ?? 0.55 to 1.74 ?? 0.24 (mean of 6, SD). An apparently spontaneous first order conversion from the PB to NPB form, with an apparent half-life of about eight days, was demonstrated at 37?? C in plasma and serum in vitro. The conversion was found to be temperature dependent and uninfluenced by the fibrinolytic components fibrinogen, fibrin, and plasminogen. Additions of a variety of enzymes or inhibitors did not interfere with the process. These results demonstrate that the PB form of alpha-2-antiplasmin is produced by the liver and that the NPB form is formed in the circulation. Chemicals/CAS: asparaginase, 9015-68-3; streptokinase, 9002-01-1; Antiplasmin; Asparaginase, EC 3.5.1.1; Streptokinase, EC 3.4.-