The invention provides a humanized immunoglobulin light chain comprising (i) variable region complementarity determining regions (CDRs) from the 3D6 immunoglobulin light chain variable region sequence set forth as SEQ ID N0:2, and (ii) a variable framework region from a human acceptor immunoglobulin light chain sequence, provided that at least one framework residue is substituted with the corresponding amino acid residue from the mouse 3D6 light chain variable region sequence, wherein the framework residue is selected from the group consisting of: (a) a residue that non-covalently hinds antigen directly: (b) a residue adjacent to a CDR: (c) a CDR-interacting residue: and (d) a residue pa11icipating in the VL-VI I interface.
