A highly sensitive and specific radioimmunoassay for kinin(minimal detectable amount, 0.5 pg/tube) was applied to measure the blood kinin level. A five ml blood sample was collected with a siliconized needle and plastic syringe which contained 2.5 ml of 0.8 N-HCl. The blood kinin was extracted with butanol, following reextraction with water. According to this procedure, the mean recovery(mean•} SE) calculated from added 125I-bradykinin(500 CPM) and the known amounts of cold bradykinin were 50.4+ 0.8% and 51.1+ 2.2%, respectively. In comparison with other sampling methods in 6 normal subjects, the blood samples taken without HCl in syringes showed a higher level (24.4•} 10.1 pg/ml) than the samples with HCl (5.3•} 1.3 pg/ml). And very high levels were obtained in the plasma samples collected by the method of Talamo or Vinci (0.53•} 0.24 ng/ml and 3.5•} 1.3 ng/ml, respectively). The kinin content in blood samples taken with HCl was stable at-20• Ž for at least one month, but increased significantly at room temperature or 4• Ž for 48 hours, Blood samples were obtained from 17 normal subjects, and 3 patients with acute myocardial infarction. Blood kinin levels in the patient with acute myocardial infarction, 121±20. g Pg/ml, were significantly higher than those in normal subjects (3.8±

0.5 pg/ml). From these results, it was concluded that high levels of blood kinin reported previously may have resulted from inadequate sampling procedures. Thus, in order to measure blood kinin accurately, inactivation of the kinin generating and destroying enzymes must be done immediately after the sampling. In addition, this radioimmunoassay method should be very useful in investigating the pathophysiological role of blood kinin in various diseases.

Kinin, produced in various organs and circulating blood (Fritz et al., 1977; Orstavik et al., 1980; Abe, 1965; Iwanaga et al., 1977) is a vasoactive peptide which has an action on smooth muscle contraction in some organs, induces blood pressure fall by vasodilatation and augments capillary permeability and pain production (Fox et al., 1961; Elliott et al., 1959; Elliot et al., 1960; Armstrong, 1970).