Kininogen was isolated from Cohns fraction IV by DEAE-chromatography, gel filtration and ammonium sulphate precipitation. Immunologically pure kininogen was prepared by removal of protein impurities using specific immunoadsorbents with Sepharose-bound antibody. Anti-kininogen serum was raised in rabbits against the pure antigen. Comparison with anti-kininogen sera prepared with the biologically active LMW antigen from whole plasma suggested antigenic identity by double immunodiffusion analysis. The Cohn-kininogen was shown to contain mainly two components (85%) in about equal amounts focusing with peaks at pI 4.2 (42%) and pI 4.3 (43%). These represent apparently structurally altered forms of the native plasma kininogen focusing at pI 4.5-4.6 (54%), which occurred as a minor component (13%).