The present study was undertaken to quantify the circulating kinins in patients with various cardiovascular diseases using a newly developed radioimmunoassay technique and to evaluate this method in terms of its clinical application. For the determination of bradykinin (BK), this assay uses a rabbit anti-serum which has been injected with kallidin. This assay shows good specific activity, recovery and reproducibility. In order to avoid the formation of kinin as well as to block its inactivation, human blood samples were collected with a polypropylene syringe containing an inhibitor mixture (EDTA, trasylol, 1-10-phenanthroline, soybean trypsin inhibitor, polybrene). 1) The plasma BK concentration in normal human subjects, in patients with essential hypertension, effort angina and other cardiac diseases were 12.2, 9.2, 8.0 and 14.0 pg/ml, respectively. 2) Thirty min after captopril (12.5 mg, p.o.) administration, blood pressure and pulmonary wedge pressures decreased, and cardiac output increased accompanied with increases in plasma renin activity, plasma BK concentration and plasma norepinephrine concentration. 3) During the cold pressor test, both plasma BK concentration and blood pressure increased in the normal human subjects, whereas plasma BK levels decreased and blood pressure increased in the patients with hypertension. This radioimmunoassay for plasma BK determination makes it possible to measure plasma BK concentration in patients with various cardiac diseases.