A simple and accurate method for the determination of kinin in blood was described. It consists of the extraction of kinin with n-butanol at pH 2.0, elimination of sodium chloride from butanol phase, re-extraction of kinin into distilled water, concentration under reduced pressure, neutralization and assay on the isolated guinea-pig ileum against synthetic bradykinin as a standard. The recovery of added bradykinin was approximately 50 per cent. The method is also specific to kinin in blood and suitable for the clinical use. The kinin level estimated by this method was about 0-2 mμg per ml in normal persons.