A prokallikrein was purified 1600-fold from rat pancreatic tissue in an overall yield of 40% by a simple four-stage procedure. The final and crucial step was immunoaffinity chromatography utilizing antibody raised to a very small amount of prokallikrein. Both the pure zymogen and the active kallikrein generated from it by trypsin activation are single chain species with Mr values of 38400 +/- 300 and 35500 +/- 400, respectively. Valine is the N-terminal amino acid residue of prokallikrein. The zymogen was comparatively stable both to autoactivation and denaturation with respect to temperature and pH. The kallikrein produced by trypsin activation of the zymogen was similar in some of its catalytic properties to the kallikrein purified from autolyzed rat pancreas but the two species differed in their susceptibility to substrate activation.