The mammalian placenta synthesises many varied antigens, including proteins, such as hormones, enzymes and protease inhibitors. In this report, we isolated and purified the two protein isomerase-related protein precursor ERp72 isoforms from aqueous extracts of guinea pig placenta, by four (4) chromatographic procedures; i) affinity chromatography on immobilised heparin, ii) gel filtration (Ultrogel AcA-54), iii) anion exchange chromatography (Mono-Q), and, iv) negative immunoaffinity chromatography. From 20 term placentae, the final yield of ERp72 isoforms was 2.4mg (Mr 71.5 kDa) and 1.5mg (Mr 75.8 kDa). Identity was confirmed by NH2-terminal amino acid sequencing which demonstrated 85% homology to human ERp72. By indirect immunofluorecence. ER p72 expression was demonstrated in tunicamycin stressed pre-implantation embryos and unfertilised oocytes. These findings demonstrate the potential for immunological monitoring of ERp72 expression, by cultured oocytes and embryos, during manipulation by assisted reproductive technologies.