J539 is an IgA myeloma protein whose V-region amino acid sequences and ligand binding for β-1.6-galactosyl oligosaccharides have been determined in the past. Two hypothetical, three-dimensional models of this protein have now been constructed on a computer display system by modifying the known structures of McPC603. The J539 Fv is 11 amino acids shorter than McPC603, and its χ-carbon backbone was formed by eliminating positions from McPC603 and rejoining the chains at three deletion sites in CDR-1 of V L and CDR-2 and -3 of V H. This maneuver was performed using a three-dimensional viewing system that displayed the amino acids with side chains in the regions around the break sites. The side chains of the amino acids in the model were oriented by a computer program that searched the available space for positions that did not make van der Waals contacts with neighboring atoms. β-1,6-Galactohexaose has been fitted to the J539 combining site using the computer display system. The terminal non-reducing sugar made close contacts with hydrogen bond accepting amino acids in CDR-2 of V H and oxygens of the third and fourth sugars were close to hydrogen bond accepting amino acids in CDR-1 and CDR-3 of V L. While the proposed structures are not unique solutions for J539 they present working models for further immunochemical experiments.