Abstract: Disclosed herein are monoclonal antibodies specific for factor XI (fXI) that prevent activation of fXI by factor XIIa (fXIIa). The monoclonal antibodies are universal fXI antibodies, capable of binding all mammalian species tested. The anti-fXI monoclonal antibodies prolong clotting time in mammalian plasmas. Moreover, administration of the fXI monoclonal antibodies disclosed herein results in inhibition of thrombosis without altering hemostasis in animal models of thrombosis. Thus, provided herein are monoclonal antibodies specific for fXI that block activation of fXI by fXIIa, compositions and immunoconjugates comprising such antibodies and their methods of use.