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.