The monoclonal antibody (MAb) 5T4 defines a human trophoblast antigen marker with a restricted pattern of expression in normal adult tissues but this antigen is expressed on a variety of carcinomas. The purification of 5T4 antigenic molecules is described from term syncytiotrophoblast by a combination of lectin- and immunoaffinity chromatography and gel filtration giving up to 10,000-fold purification with 70% yield. The antigen is carried by non-associated glycoprotein molecules with an apparent molecular weight of 72 kDa on SDS-PAGE and a neutral pI. Removal of N-linked sugars by N-glycanase reveals a core protein of 42 kDa. Treatment with enzymes that cleave O-linked sugars does not substantially alter the molecular size. The native 5T4 molecules are very resistant to proteolysis until the N-linked sugars are removed or the glycoprotein is denatured and reduced. Glycopeptides generated by these approaches will be suitable for amino acid sequencing.