Stearyltrimethylammonium chloride was used to isolate human fibrinogen, and purified protein was obtained by removing the detergent bound to it. Medium consisting of 0.015–0.03 mM fibrinogen-detergent complex, 0.85 M NaCl, 0.03 M sodium caprylate, and 30% ethanol was found to be effective for renaturation of fibrinogen from the complex. The purified fibrinogen did not form any fibrils on incubation for 15 days with Ca+2 at pH 7.2, and 37°. The clottability of the purified fibrinogen was over 99%. Immunochemical studies showed that the purified fibrinogen produced one precipitation line with a mixture of anti-human fibrinogen and anti-human serum. Although highly purified, the fibrinogen preparation still contained a trace of plasminogen.