The precursor of the kinin-forming enzyme, prekallikrein, was isolated from rabbit plasma protected from activation during preparatory procedures. Prekallikrein was shown to be a 4.5S gamma(1)-glycoprotein with an isoelectric point of 5.9 and a mol wt of 99,900. The proenzyme was activated at neutral pH by an enzyme from rabbit or human plasma we have termed prekallikrein activator (PKA) or by trypsin. Prekallikrein was activated by PKA by a process of enzymatic scission. This resulted in the appearance of two fragments; the larger of these possessed kallikrein activity.