Abstract: In a method for assaying an anticardiolipin antibody in a sample utilizing .beta.2-glycoprotein I, a polypeptide having the same amino acid sequence as domain IV of .beta.2-glycoprotein I or a polypeptide partially different therefrom but functionally equivalent thereto is used in place of .beta.2-glycoprotein I itself. According to this method, an autoantibody from patients with antiphospholipid syndrome can be accurately assayed in a simple manner.

Abstract: As a carrier for binding of the antiphospholipid antibodies used for immunological diagnosis of antiphospholipid syndrome, a phospholipid-bound carrier treated with purified serum albumin and a surfactant is used. Thus, immunological diagnosis of antiphospholipid syndrome can be made with high accuracy. By using the fraction or protein obtained from animal serum or plasma, having the activity of enhancing the binding ability of the antibodies specifically present in the antiphospholipid syndrome to the phospholipid, immunological diagnosis of antiphospholipid syndrome can also be made more accurately, as compared to known diagnosis.

Abstract: A solid phase reagent is provided which comprises a carrier having a surface onto which functional groups containing a negative charge or a lone pair of electrons and/or free radicals containing a negative charge or a lone pair of electrons have been introduced and having .beta.2-glycoprotein I coated on the surface. Using this reagent, antibodies specific to antiphospholipid syndrome can be specifically assayed. Moreover, antibodies specific to antiphospholipid syndrome can be assayed differentially from antibodies specific to infectious diseases by using the solid phase reagent described above or a further modified solid phase reagent.