A turbidimetric assay for thrombolytic enzyme was performed in microtiter assay plates, wherein decrease in light scatter at 655 nm of fibrin plates in individual wells at 37℃ was recorded automatically at different times and used as a quantitative measure of fibrin dissolution. Use of an automated microtiter plate reader facilitated this assay considerably and allowed many assays to be performed rapidly, with a large amount of replication. The reliavility of the –ΔOD_<655>, CV%, caluculated by subtracting OD_<655> at individual indicated incubation time in eight microtiter wells from OD_<655> of microtiter wells before adding “natto” extract was found to be in 10% throughtout various degrees of dilution of “natto” extract. Logarithmic values of the degree of dilution of “natto” extract and –ΔOD_<655>/hr showed a linear relation, irrespective of the sample quality, indicating that this is a good method for the determination of thrombolytic activity in “natto” products.