A method using immunoadsorbents for the isolation of pregnancy-associated alpha2-glycoprotein (alpha2-PAG) from the extract of human placentae is described. The physical properties and the chemical composition of the purified protein are determined: alpha2PAG sediments with 11,5 S, has a molecular weight of 360 000 daltons and is composed of subunits having a molecular weight of 180000, which are held together by disulfide bonds. The isoelectric point was found to be pH 4,7 and the extinction coefficient (E1%1cm) was determined to be 9,7 at 277 nm. The carbohydrate content of the molecule amounts to 12,1% (hexose 6,0%, hexosamine 3,7%, fucose 0,06%, sialic acid 2,4%). An analysis of the amino acids is reported, too. The purified alpha2PAG was used to determine the absolute concentrations of this protein in a reference standard and in sera.