Prokallikrein was partially purified from porcine pancreas by water extraction in the presence of soybean trypsin inhibitor, ethylenediaminetetraacetic acid (EDTA), N-ethylmaleimide and benzamidine, followed by a combination of ammonium sulfate fractionation, ion-exchange chromatographies, gel filtration and chromatofocusing in the presence of various protease inhibitors. The prokallikrein preparation obtained was stable on storage at -25°C ; after 1.5 months only 2% of prokallikrein was activated. It was also fairly stable when it was lyophilized and stored at room temperature. The prokallikrein was rapidly activated by trypsin. Chymotrypsin also activated prokallikrein, but an extremely large amount of chymotrypsin was necessary. Urokinase, cathepsins C and D, plasmin, thrombin, Factor Xa, collagenase, elastase, prolidase and leucine aminopeptidase caused no detectable activation of prokallikrein. Several components of prokallikrein were detected by isoelectric focusing. The pI values of the two main components were 4.47 and 4.68. Slight spontaneous activation of prokallikrein occurred during the chromatographies even when the greatest care was taken.