b₂-Glycoprotein I (b₂GPI) is a plasma apolipoprotein with anticoagulant and other antiatherogenic properties. Human protein sequence, as well as secondary and terciary structures, is described. There are some reports on dimeric and multimeric forms. Macroaggregation was observed in vitro and is suspected also to occur in vivo. Both dimerization and macroaggregation were proposed to favor structural modification, through conformational changes and fragment loss. Despite controversial data in the literature, monoclonal antibodies with I, IV and V domain specificity were reported not to recognize hydrogen peroxide oxidized forms and protein dimers. Small subunits were proposed in the literature, but macroaggregate instability and heterogeneity precluded the characterization of these molecules. Research groups usually purify b₂GPI from human or animal plasma, and keep their purified protein preparations frozen for a short period of time, avoiding multiple freeze-thaw cycles, in order to prevent oxidation and dimerization interference. Commercial standards are expensive and present inhomogeneous SDS-PAGE patterns, due to modified protein contamination. In searching of an improved stability standard for our laboratory work, we lyophilized b₂GPI from aqueous solutions. Frozen aliquotes from the same b₂GPI preparations were used as controls. b₂GPI was isolated from human plasma pools as previously described, by heparin-sepharose affinity chromatography after perchloric acid precipitation. Dried and frozen preparations were studied according their SDS-PAGE profiles and competitive ELISA reactivity. We could demonstrate dimers (97-120 kDa bands) appearing after a 3-month freeze storage, but not in lyophilized b₂GPI. Lyophilized b₂GPI presents only the native b₂GPI monomer bands in SDS-PAGE (multiple, from 54 kD through 66kDa). MoAb 5F7 recognizes the 45kDa b₂GPI band, as well as native freshly isolated b₂GPI in immunoblots. Storage modified profiles could not be obtained after hydrogen peroxide oxidation of the isolated protein. Taken together, these results support previous data suggesting that protein oxidation is not the main or even earliest structural modification during protein storage in aqueous solutions. They also demonstrate that at least early b₂GPI storage-associated modifications can be circumvented by lyophilization of the aqueous solutions, improving standardization (FAPESP, CNPq).