Antithrombin from bony fish (Teleostei), represented by an ancient salmonid, Atlantic salmon (Salmo salar L.), and a more evolved species from the same family, rainbow trout (Oncorhynchus mykiss Walbaum), functions in vitro as does its human counterpart: it inactivates thrombin almost instantaneously in the presence of heparin and only slowly when heparin is absent. The inhibitory activity of salmonid antithrombin towards the homologous thrombin did not differ noticeably from its inactivating capacity in heterologous (teleost) systems, and enzyme-inhibitor reactions between reagents from fish and man proceeded just as efficiently. In all enzyme-inhibitor reactions with salmonid thrombin the inactivation by salmonid antithrombin or diluted fish plasma was maximal at pH 7.8-8.4. The inactivation was clearly dependent on heparin in all systems and maximal at concentrations between 1.5 and 6 U/ml. What particularly distinguishes the salmonid thrombin-antithrombin interaction from the human one is that the former has to function over a wide range of temperatures. And the thrombin inactivating capacity of purified antithrombin and diluted plasma in the presence of heparin was indeed present at temperatures down to 3 degrees C, a capacity that human antithrombin also has retained. Even more interesting was that the teleost enzyme-inhibitor reaction was nearly independent of temperature under the conditions studied.