Our objective was to characterize monoclonal antiphospholipid antibodies (APL) and identify disease-associated antigens in patients with the antiphospholipid syndrome (APS). We used the monoclonal antibody HL-5B, derived from a patient with APS suffering from multiple ischemic events, to screen a 12-mer peptide phage display library (New England Biolabs, London, England). The identified phage clones were sequenced and the derived consensus peptide was synthesized. The peptide was used to perform competitive inhibition experiments for their ability to inhibit the binding of the monoclonal antibody and of serum antibodies to cardiolipin and phosphatidylserine. Additionally patients and control sera were screened for their binding reactivities to this peptide. Using this 12-mer phage display library the peptide APHKHKASLSIY as consensus peptide for the monoclonal antiphospholipid antibody HL-5B could be identified. In competitive inhibition studies we showed that this peptide is able to inhibit the binding of HL-5B to cardiolipin and phosphatidylserine and furthermore another antiphospholipid antibody used as control was also inhibited in its binding to phospholipids. Using 21 sera from APS patients 67% showed a binding to the peptide in a specific ELISA above the cutoff level, generated with sera from 20 healthy controls. Out of the reactive patients' sera we used two exemplarily to perform inhibition studies. Both sera could be inhibited more than 40% in their binding to cardiolipin in a commercially available antiphospholipid antibody assay (Aescu.diagnostics, Wendelsheim, Germany). The identified peptide APHKHKASLSIY simulates the antigenic structure recognized from a subpopulation of serum antiphospholipid antibodies. This might indicate that the diversity of the antiphospholipid antibodies is limited and only few epitopes or few common structures are responsible for the development of those antibodies. Tests using these epitopes will strongly improve laboratory diagnosis of the APS.