Human alpha- and gamma-thrombins (with high and essentially no fibrinogen clotting activities, respectively) were inhibited in chromogenic substrate assays by the dipeptidyl argininals: antipain less than leupeptin less than H-D-Phe-Pro-argininal approximately Boc-D-Phe-Pro-argininal. In clotting assays with alpha-thrombin, I50 values were slightly higher than Ki values from chromogenic substrate assays, except for a somewhat lower I50 for antipain. Our data cautions the use of argininal proteinase inhibitors in the assessment of thrombin functions, and the high potency of H-D-Phe-Pro-argininal and its derivative suggest pharmaceutical applications.