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13F6-1-2 is a murine monoclonal antibody that recognizes the heavily glycosylated mucin-like domain of the Ebola virus virion-attached glycoprotein (GP) and protects animals against lethal viral challenge. Here we present the crystal structure, at 2.0, of 13F6-1-2 in complex with its Ebola virus GP peptide epitope. The GP peptide binds in an extended conformation, anchored primarily by interactions to the heavy chain. Two GP residues, Gln P406 and Arg P409, make extensive side-chain H-bond and electrostatic interactions to the antibody and are likely critical for recognition and affinity. The 13F6-1-2 antibody utilizes a rare V lambda(sub x) light chain. Surprisingly, the three CDR light chain loops do not adopt canonical conformations and represent new classes of structures distinct from V kappa and V lambda light chains. The light chain makes five hydrogen bonds to the peptide, but interestingly, all contacts are mediated through germ line-encoded residues. The 13F6-1-2 V lambda (sub x) light chain shares strong sequence identity to human V lambda subgroup VIII, thus providing a framework for humanization. This first structure of a Vx light chain and Ebola virus-neutralizing antibody is an exciting step towards the development of a postexposure therapeutic antibody.