Methods of determining thrombolytic activity of nattokinase (NK), a thrombolytic enzyme from Natto (a fermented soybean product in Japan), were studied. When the activity was determined by measuring lysis area of artificial thrombus (fibrin) on plates at 37°C, values were markedly influenced by incubation time and various substances present in crude samples. Then, NK activity was measured by estimating the lysis time of fibrin clot prepared in test tubes. Logarithmic values of the clot lysis time (CLT) and NK activities of the standard NK showed a linear relation, irrespective of the sample quality, indicating that this is a good method for the determination of NK activity. When NK activities of various Natto samples were determined by the CLT method, values obtained ranged from 3, 000 to 24, 000 units/100 g of Natto (wet weight). These results ehowed that NK activity was greatly varied depending on the products even if they were made with the same species of Bacillus.