The complexes between the Fab fragments of two monoclonal anti-lysozyme antibodies, Fab10. 6.6 (high affinity) and D44. 2 (lower affinity), and their specific antigen, hen egg-white lysozyme, have been crystallized. The antibodies recognize an antigenic determinant including Arg68, but differ significantly in their association constants for the antigen. Two crystalline forms were obtained for the complex with FabF10. 6.6, the higher affinity antibody. One of them is monoclinic, space group P2 1, with unit cell dimensions a= 145.6 A ̊, b= 78.1 A ̊, c= 63.1 A ̊, β= 89.05°, consistent with the presence of two molecules of the complex in the asymmetric unit. These crystals diffract X-rays beyond 3 Å making this form suitable for high-resolution X-ray diffraction studies. The second form crystallizes in the triclinic space group P 1, with unit cell dimensions a= 134.0 A ̊, b= 144.7 A ̊, c= 98.6 A ̊, α= 90.30°, β= 97.1°, γ= 90.20°, consistent with the presence of 10 to 12 molecules of the complex in the unit cell. These crystals do not diffract X-rays beyond 5 Å resolution. The antigen-antibody complex between FabD44. 2, the lower affinity antibody, and hen egg-white lysozyme crystallizes in space group P2 1 2 1 2 1, with unit cell dimensions a= 99.7 A ̊, b= 167.3 A ̊, c= 84.7 A ̊, consistent with the presence of two molecules of the complex in the asymmetric unit. These crystals diffract X-rays beyond 2.5 Å resolution.