We attempted to identify the presence of kallikrein in human vascular tissue obtained from patients undergoing surgery. Sections of thoracic (n = 9) and abdominal aorta (n = 6), renal artery (n = 6), and saphenous vein (n = 17) were rinsed with 0.01 mol/L Tris-HCl buffer, cleaned, minced, and homogenized at 4°C. The homogenates were centrifuged and supernatants were assayed for protein content and for active and total (trypsin activation) enzymatic activity on the peptide H-D-Val-Leu-Arg-paranitroanilide (S2266), a synthetic substrate for glandular kallikrein. Enzymatic activity was inhibited by aprotinin and polyclonal antibodies against human glandular kallikrein. Kallikrein was resistant to soybean trypsin inhibitor and had an optimum pH of 8.2. A significant correlation was found between the amidolytic and kininogenase activities measured on S2266 and dog kininogen, respectively (r = 0.83, P < .01). The kallikrein-like enzyme was present mainly in the inactive form. Higher levels were found in the homogenates of renal artery (active: 190 ± 36, total: 5036 ± 908 pkat/g protein) than in those of thoracic (active: 38 ± 9, total: 973 ± 350 pkat/g protein) and abdominal aorta (active: 44 ± 10, total: 3031 ± 709 pkat/g protein). In the homogenates of saphenous vein, active and total enzymatic activities averaged 188 ± 90 and 2003 ± 450 pkat/g protein, respectively. A significant inverse correlation was found between the levels of total enzymatic activity in saphenous vein homogenates and mean blood pressure values (r = 0.78, P < .005). These results suggest that a kallikrein-like enzyme is present in human vasculature. Vascular kallikrein might act as a local hormone and regulate regional hemodynamics by releasing kinins. Am J Hypertens 1993;6:344–348