In order to produce a high-titred antiserum against factor IX a sheep was immunized with highly purified human factor IX. One ml of the antisera obtained from three different bleedings neutralized 90% of the activity of factor IX in 130–200 ml standard plasma. The antisera gave one main precipitin band and occasionally an additional weak precipitin band against normal plasma in double immunodiffusion. The purified factor IX preparation had a specific activity of 290 U per absorbancy unit at 280 nm. However, it was not homogenous in various polyacrylamide gel electrophoresis techniques. In analytical polyacrylamide gel electrophoresis one main band and several additional weaker bands with lower electrophoretic mobility were seen. When this gel was submitted to electrophoresis into an agarose gel containing the antiserum against factor IX, one precipitin arc was seen corresponding to each of the bands. Reactions of identity was seen between the precipitin arcs, thus demonstrating the presence of factor IX antigen in all the bands. The minor bands with lower electrophoretic mobility disappeared on polyacrylamide disc gel electrophoresis in the presence of 10M urea and on SDS polyacrylamide electrophoresis. They may represent aggregates of factor IX, or complexes between factor IX and other proteins.