Objective. To characterize the binding and functional properties of antiphospholipid antibodies (aPL) induced by immunization with a viral peptide and to determine whether aPL are pathogenic in vivo.Methods. Ten murine monoclonal aPL were generated from spleen cells of PL/J mice immunized with TIFI, a phospholipid-binding peptide spanning Thr 101 -Thr 120 of ULB0-HCMVA from human cytomegalovirus (CMV), which shares structural similarity with the phospholipid-binding site of  2 -glycoprotein I ( 2 GPI).Results. The antibodies generated had aPL activity that was inhibited by cardiolipin liposomes, and this inhibition was enhanced in the presence of  2 GPI. Some of the antibodies exhibited binding to cultured endothelial cells in vitro, and some had lupus anticoagulant activity. Injection with 2 of the monoclonal aPL in mice resulted in a significant increase in the number of leukocytes adhering to endothelial cells and enhanced thrombus formation in vivo.Conclusion. These results indicate that aPL induced by immunization with a phospholipid-binding CMV peptide are pathogenic in vivo. The results also suggest a mechanism (molecular mimicry) by which pathogenic aPL may be generated in patients with antiphospholipid syndrome.