X-ray crystal structures of fragments from two different humanized anti-CD18 antibodies are reported. The Fv fragment of the nonbinding version has been refined in space group C2 with a = 64.2 A, b = 61.3 A, c = 51.8 A, and beta = 99 degrees to an R-value of 18.0% at 1.9 A, and the Fab fragment of the tight-binding version has been refined in space group P3 with a = 101. A and c = 45.5 A to an R-value of 17.8% at 3.0 A resolution. The very large difference in their binding affinity (> 1000-fold) is attributed to large and local structural differences in the C-terminal part of CDR-H2, and from this we conclude there is direct contact between this region and antigen when they combine. X-ray structures of antibody-antigen complexes available in the literature have yet to show this part of CDR-H2 in contact with antigen, despite its hypervariable sequence. Implications of this result for antibody humanization are discussed.