Considerable interest is currently focused on the interactions of beta-2 glycoprotein I (β2GPI) and anti-phospholipid antibodies with anionic phospholipids in an attempt to understand the association between these antibodies and clinical diseases such as thrombosis. The interactions of β2GPI and anionic phospholipids have only been characterized partially, and the physiological role of this glycoprotein remains uncertain. In this study we have explored in detail the physical and phospholipid-binding characteristics of a number of β2GPI preparations. We have found (i) that perchloric acid-purification methods are damaging to β2GPI during purification, (ii) that the dissociation constants of the various preparations for phosphatidylserine vary between 0.1-2 μM and are considerably weaker than previously reported, (iii) that considerable differences in affinity of the various β2GPI preparations for anionic phospholipids are obtained when comparing anionic phospholipids immobilized to a solid-phase versus phospholipid assembled in unilamellar vesicles, (iv) that the integrity of the fifth domain of β2GPI is important for binding immobilized anionic phospholipid but not especially important in binding vesicular anionic phospholipid, and (v) that β2GPI preparations with differing isoelectric species content bind anionic phospholipids differently, suggesting that varying glycosylation and/or protein polymorphisms impact upon phospholipid binding. These results highlight the importance of assessing the determinants of the interaction of β2GPI with anionic phospholipids assembled in unilamellar vesicles.