Several factors having an influence on the thrombin activity were investigated, and the following two results were obtained : 1) Inactivation of thrombin activity apparently depended on not only incubation time of solution, but also the amount adsorbed on the glass surface of the test tube. 2) Thrombin (J.P.) solution, being incubated at 4°C, showed the temporary increase of thrombin activity, and this phenomenon might be attributed to the conversion of contained prothrombin to thrombin.
