AbstractThe three‐dimensional structure of the human immunoglobulin fragment Fab New (IgG1,λ) has been refined to a crystal‐lographic R‐factor of 16.9% to 2Å resolution. Rms deviations of the final model from ideal geometry are 0.014 Å for bond distances and 3.03° for bond angles. Refinement was based on a new X‐ray data set including 28,301 reflections with F>2.5σ(F) from 6.0 to 2.0 Å resolution. The starting model for the refinement procedure reported here is from the Brookhaven Protein Data Bank entry 3FAB (rev. 1981). Differences between the initial and final models include modified polypeptide‐chain folding in the third complementarity‐determining region (CDR3) and the third framework region (FR3) of VH and in some exposed loops of CL and CHl. Amino acid sequencechanges were determined at a number of positions by inspection of difference electron density maps. The incorporation of amino acid sequence changes results inan improved VH framework model for the “humanization” of monoclonal antibodies.