A prokallikrein was isolated from bovine pancreas by a multi-step procedure involving gel filtration, hydrophobic interaction and anion-exchange chromatographies. The purification was initially monitored by measurement of the kinin-releasing activity of the activated zymogen. Later, when the pure prokallikrein had been isolated, a specific radioimmunoassay for the zymogen was set up and that was employed to provide estimates of 323-fold and 28% for the overall degree of purification and percentage recovery of prokallikrein. The relative molecular weight of prokallikrein was found to be 26,900 by SDS gel electrophoresis and its isoelectric point was established as pH 4.55.