H-D-Phe-Pip-Arg-pNA (S-2238) hydrochloride, a chromogenic substrate, is patterned after the N-terminal portion of the A alpha chain of fibrinogen, which is the natural substrate of thrombin. H-D-Phe-Pip-Arg-pNA hydrochloride is specific for thrombin and is used to measure antithrombin-heparin cofactor (AT-III). The AT-III assay using H-D-Phe-Pip-Arg-pNA hydrochloride is sensitive, accurate, and easy to perform.