Summary A new activated factor IX product (Autoplex) has been used for the treatment of 18 haemarthroses in three haemophiliac patients with antibodies to factor VIII and a history of high anamnestic response to challenge with factor VIII. Results were evaluated by assessing pain, tenderness, range of movement and girth at 8 and 24 hours. A score of 2 was given for improvement at 8 hours and 1 for improvement at 24 hours. The result was expressed as a percentage of the possible score. Nine episodes treated with less than 35 units of Autoplex/kg had a mean score of 52% and nine episodes treated with more than 35 units/kg had a mean score of 73%. When only the more severe group of bleeds was considered, an even more marked dose related response emerged. When the ratio of the percentage fall in the PT to the percentage fall in the KCCT was plotted against the outcome, a significant correlation emerged (r=0.43; P= 0.02).