HAPC1573 (1573) antibody treatment significantly improves survival and ameliorates joint bleeding in a knee-joint injury model. The capsules of the right knee joints of anesthetized mice were punctured under the patella with a 30 × 0.5-G needle to induce joint bleeding. Knee diameter, before the needle injury and every other day during the 2 weeks after the injury, was measured with an electronic caliper, and the change of joint diameter was calculated as percentage of the before-injury value. (A,C) The survival rates of mice after the injuries. Survival curves were plotted using the Kaplan-Meier method. (B,D) Changes of knee bleeding represented as percentage of the knee diameter change. Data were expressed as mean ± SD. PROC^+/+;F8^−/− mice were pretreated for 30 minutes with either 2 IU per mouse of FVIII or 20 µg per mouse of 1573 (A-B). PROC^+/+;F9^−/− mice were pretreated for 30 minutes with either 2 IU per mouse of FIX or 20 µg per mouse of 1573 (C-D). **P < .01 (log-rank test).