A mouse IgA monoclonal antibody, MoAb 117,C-11, was produced against the glucose/mannose specific isolectin I (LoLI) from Lathyrus ochrus seeds, a legume two-chain lectin comprising two heavy subunits non covalently associated to two light subunits. The MoAb reacted in ELISA with three different peptides representing three distinct amino acid stretches present on the surface of the heavy subunits of LoLI. Since these three stretches are too far apart to build a discontinuous/conformational epitope, it is presumed that they act as continuous/sequential epitopes. The presence of a complete or partial TGNV sequence within these epitopes could account for their reactivity towards the MoAb, as shown by inhibition studies using a chemically synthesized TGNV peptide. MoAb 117,C-11 also reacted with closely related two-chain lectins.