Existing views on prorenin are conflicting and its physiological activation mechanism is not clear. In an attempt to obtain clearcut views on the molecular properties of prorenin in human plasma, the renin zymogen (prorenin) was separated from active renin by two steps of affinity chromatography and it was demonstrated that prorenin is a completely inactivate zymogen contrary to the existing information. Inactive prorenin has an apparent molecular of 56,000 contrary to 46,000-43,000 of partially active prorenin. Isolated and acid-treated human prorenin was shown to be activated by kallikreins from human urine and plasma. This activation was completely blocked by Trasylol. Hog pancreatic kallikrein also activated human prorenin. The kallikrein mediated activation of prorenin indicates the existence of a new link between the vasoconstricting renin-angiotensin system and the vasodilating kallikreinkinin system.