The invention relates to a method for determining the concentration of thrombin inhibitors in a non-turbid body liquid or a non-turbid extract from a body liquid. It comprises the following steps. The body liquid is taken from a living body, and the body liquid is subjected to a separation from the turbid matter, if necessary. To the non-turbid body liquid thus obtained are added a coagulation-inhibiting substance not interfering in the transformation prothrombin/active meizothrombin or Mtdesfg1, resp., a chromogenic or fluorogenic substrate not dissociable by active meizothrombin or Mtdesfg1, resp., and a substance dissociating prothrombin into meizothrombin or Mtdesfg1, resp., and as an option prothrombin. The thus obtained solution or mixture, resp., is subjected to a wavelength-selective light absorption or light emission measurement as a function of the time. From the reduction of the light absorption or light emission per time unit is determined the amount of the thrombin inhibitor included in the body liquid by comparison to previously determined standard curves.