LewisX-containing glycoconjugates are abundantly expressed by schistosomes and are assumed to be of prime importance for the survival of the parasite within the human host. Monoclonal antibody 291-2G3-A, which was generated from mice infected with schistosomes, was found to interact with monomers, dimers and trimers of the LewisX trisaccharide. The Fab fragment of monoclonal antibody 291-2G3-A has been crystallized and soaked with its LewisX antigen. X-ray data sets were recorded for the different Fab crystals with and without LewisX. Crystals grown from 25% polyethylene glycol 3350, 0.17 M ammonium sulfate and 15% glycerol belong to the triclinic space group P1, with unit-cell parameters a = 67.4, b = 71.6, c = 104.8 A, alpha = 86.5, beta = 71.3, gamma = 83.3 degrees for the native crystals and with slightly different unit-cell parameters a = 67.3, b = 72.4, c = 104.8 A, alpha = 85.8, beta = 71.3, gamma = 83.3 degrees for the crystals containing bound LewisX. Crystals grown from 14% PEG 3350, 50 mM Tris pH 8 and soaked with LewisX also belong to the triclinic space group P1, but with different unit-cell parameters a = 45.1, b = 60.8, c = 91.6 A, alpha = 96.0, beta = 95.4, gamma = 101.8 degrees.