Abstract. Human factor II was isolated from plasma after absorption and elution of the prothrombin complex with barium citrate and ammonium sulphate, respectively. The eluate was brought onto a QAE Sephadex A‐50 column. The fractions containing coagulation factors were pooled, concentrated and chromatographed on hydroxyapatite, after which the fractions containing factor II activity were used for the immunization of rabbits. The antiserum produced is monospecific for factor II, as could be ascertained by immunoprecipitation and by inhibition of the coagulation activity.