The α(1)-antitrypsin (α(1)AT), which comprises more than 90 % of the protein content of the α(1)-globulin, inhibits a variety of enzyme. α(1)AT is the major trypsin inhibitor in human serum and has an important role as antiplasmin in fibrinolytic system. The experiments were carried out to investigate the effect of α(1)AT on fibrinolytic system, measuring serum α(1)AT levels and coagulative and fibrinolytic activities in man. The serum α(1)AT levels were measured in 30 controls, 93 cancer and 107 nonmalignant patients. They were also measured in the postoperative patients who were 7 cases of gastric cancer, 5 of lung cancer, 5 of breast cancer and 5 of cholelithiasis. The serum levels were measured by a single radial immunodiffusion plate (M-partigen) obtained from the Behring Institute. The results obtained were as follows: 1) The levels of α(1)AT were significantly increased in patients with lung cancer, gastric cancer and colon cancer, while those with breast cancer and thyroid cancer remained in normal range. There was a significant correlation between α(1) AT and the clinical stage of lung cancer and gastric cancer. Among nonmalignant patients, AT was significantly elevated in the inflammatory diseases such as lung abscess and hepatitis. 2) α(1)AT had the lowerest level at the end of operation and it exceeded the preoperative value and returned to preoperative level at the 14th day. These changes of α(1)AT were observed both cancer and nonmalignant patients. The plasma fibrinogen levels showed a similar pattern to α(1)AT during postoperative period, whereas d(2)-macroglobulin had no significant changes. The concentration of plasma plasminogen and antithrombin III had been fallen below the preoperative levels by the 7th day and returned at the 14th day. Antitrypsin activities were increased to some extent by the 7th postoperative day. These results suggest that the elevation of α(1)AT is closely related to two factors as follows: a) α(1)AT may increase to neutralize a certain leucocytic protease when leucocytes increase in body. b) The increase in α(1)AT might be due to regulating the plasmin which auguments in cancer, inflammation or postoperative patients, may induce release of antiplasmin whose activity is mainly dependent on α(1)AT.