Human fibrinogen was purified free of plasminogen by a simple immunoadsorption technique using an insoluble polymer complex of an anti-plasminogen antiserum cross-linked by ethylchloroformate. The anti-plasminogen antiserum was prepared in guinea pigs by immunization with purified human plasminogen. The fibrinogen so purified was free of plasminogen as demonstrated by immunological and biological assays and did not show changes in clottability, thrombin clotting time and reactivity to plasmin, and is suitable as a reagent for plasmin and anti-plasmin assays for diagnostic purposes.