The present invention relates to the provision of antibody fragments capable of binding selectively to the Gal-α-(1→3)-Gal epitope. The invention further relates to assay systems comprising these antibody fragments for use in testing transplantation tissue for possible rejection complications. This epitope is often found on porcine tissue destined for human transplantation. The epitope is also found on biopharmaceuticals and on some infectious agents and accordingly the invention also provides assay systems for these applications.