In patients who required volume expansion during extracorporeal circulation, the plasma bradykinin concentration was monitored simultaneously with the mean arterial pressure during infusion of either albumin solution or PPF. The PKA content of the PPF and the albumin solution was 29 and 3 U/L, respectively, measured spectrophotometrically. In six patients receiving 250 ml of PPF, the mean arterial pressure decreased 22% to 54% within 1.5 min after infusion, whereas the plasma bradykinin concentration, measured by radioimmunoassay, increased significantly (p less than 0.0005) during the first minute. In six patients receiving 250 ml of 4% albumin solution, no blood pressure changes were found, and the plasma bradykinin concentration rose only slightly. In vitro, linear correlation (r = 0.94, p less than 0.0005) was observed between the level of PKA of 26 different lots of PPF and the concentrations of bradykinin that were generated in Hageman factor-deficient plasma after incubation with PPF. It is concluded that the hypotensive reactions observed after PPF infusion during extracorporeal circulation are caused by the PKA-induced bradykinin generation.