We report an antibody (anti-TSEN) that recognizes an antigen (TSEN) at the unique amino acid sequence that results from the junction of GPA58 to GPB27 if the GPB carries the S antigen. Red cells from several unrelated donors that possess this specific GP(A-B) hybrid molecule were agglutinated by anti-TSEN. Since a synthetic peptide with the amino acid sequence at this junction (Pro-Glu-Glu-Glu-Thr-Gly-Glu-Met-Gly-Gln-Leu-Val-His-Arg) specifically inhibited anti-TSEN, it must detect an antigen within this novel amino acid sequence. The TSEN antigen has been provisionally assigned the MNS blood group system number 002.033 on behalf of the ISBT Working Party on Terminology for Red Cell Surface Antigens.