Bispecific antigen binding molecules (e.g., antibodies) that bind blood clotting factors, factor IXa (FIXa) and factor X (FX), and enhance the FIXa-catalysed activation of FX to FXa. Use of the bispecific antigen binding molecules to control bleeding, by replacing natural cofactor FVIIIa which is deficient in patients with haemophilia A.