We have identified a phospholipid binding site in the fifth domain of beta 2-glycoprotein I (beta 2-GPI). Using synthetic peptides spanning the fifth domain of beta 2-GPI, we have shown that the presence of the sequence Glu274-Cys288 caused a decrease in the binding of purified anticardiolipin (aCL) antibodies in a modified cardiolipin (CL)-ELISA by inhibiting the binding of beta 2-GPI to CL. This peptide bound to and could be eluted from a CL affinity column in a manner similar to native beta 2-GPI. Peptides corresponding to other regions of the fifth domain had no inhibitory effect. The inhibitory activity was restricted to the sequence Cys281-Lys-Asn-Lys-Glu-Lys-Lys-Cys288. Peptides in which the two flanking cysteine residues were deleted or substituted with serine residues possessed no inhibitory activity, indicating that the conformation of this highly positively charged sequence may be critical for phospholipid binding. aCL antibodies purified from patients with autoimmune disease were shown to bind directly to wells coated with native beta 2-GPI but not to wells coated with a preparation of beta 2-GPI cleaved between Lys317 and Thr318. The integrity of this sequence is therefore critical for these antibodies to recognize beta 2-GPI, and the putative epitope for aCL antibodies is most likely to be in this region.