The invention relates to compounds of general formula (I) and acid addition salts thereof, where the various symbols have the meanings given in the description and claims, the production and use thereof as substrate for the detection of TAFIa, a fibrinolysis inhibiting enzyme. The detection occurs by using the absorption between 400 and 412 nm, arising as a result of the formation of 3-carboxy-4-nitrothiophenol from Ellman's reagent as a function of time. ##STR00001##