Human high molecular weight kininogen was isolated by a rapid procedure, using anion exchange chromatography on QAE-Sephadex, ammonium sulfate precipitation and cation exchange chromatography on CM-Sephadex. The poor recovery and relatively low specific activity observed in earlier experiments was found to be due to a contaminant, presumably enzymatic, capable of releasing kinin from the kininogen. The "spontaneous" kinin release was blocked by soy bean trypsin inhibitor and by C1-inactivator. The isolated kininogen was stable at different temperatures, did not contain free kinin and was a good substrate for plasma kallikrein and plasmin.