The present inventors produced a variety of bispecific antibodies that specifically bind to both F. IX/F. IXa and F. X, and functionally substitute for F. VIIIa, i.e., have a cofactor function to promote F. X activation via F. IXa. Among these antibodies, the antibody A44/B26 reduced coagulation time by 50 seconds or more as compared to that observed when the antibody was not added. The present inventors produced a commonly shared L chain antibody from this antibody using L chains of A44, and showed that A44L can be used as commonly shared L chains, although the activity of the resulting antibody is reduced compared to the original antibody (A44HL-B26HL). Further, with appropriate CDR shuffling, the present inventors successfully produced highly active multispecific antibodies that functionally substitute for coagulation factor VIII.
