The monoclonal anti-tumor antibody BR96 binds a tetrasaccharide, Lewis y (Le(y)), in vitro and recognizes a Le(y)-bearing or Le(y)-related tumor-associated antigen in vivo. The Fab of the murine monoclonal antibody, mBR96 (IgG3, kappa), and the Fab' of its human chimera, cBR96 (IgG1, kappa), and their complexes with Le(y) have been screened for crystallization conditions. Crystals suitable for X-ray diffraction have been obtained for uncomplexed cBR96 Fab', cBR96 Fab' in complex with Le(y) and mBR96 Fab in complex with Le(y). The symmetry of the cBR96 Fab' crystals is consistent with space group P2(1)2(1)2, a = 61.1 A; b = 174.3 A; c = 45.6 A; the symmetry of the cBR96 Fab'-Le(y) complex crystals with space group P4(3)2(1)2 (or its enantiomorph), a = b = 82.2 A; c = 167.1 A and the symmetry of the mBR96 Fab-Le(y) complex crystals with space group P2(1)2(1)2(1), a = 69.4 A; b = 84.9 A; c = 86.8 A.