The murine monoclonal antibody WO2 specifically binds the N-terminal region of the amyloid beta peptide (Abeta) associated with Alzheimer's disease. This region of Abeta has been shown to be the immunodominant B-cell epitope of the peptide and hence is considered to be a basis for the development of immunotherapeutic strategies against this prevalent cause of dementia. Structural studies have been undertaken in order to characterize the molecular basis for antibody recognition of this important epitope. Here, details of the crystallization and X-ray analysis of the Fab fragment of the unliganded WO2 antibody in two crystal forms and of the complexes that it forms with the truncated Abeta peptides Abeta(1-16) and Abeta(1-28) are presented. These crystals were all obtained using the hanging-drop vapour-diffusion method at 295 K. Crystals of WO2 Fab were grown in polyethylene glycol solutions containing ZnSO(4); they belonged to the orthorhombic space group P2(1)2(1)2(1) and diffracted to 1.6 A resolution. The complexes of WO2 Fab with either Abeta(1-16) or Abeta(1-28) were cocrystallized from polyethylene glycol solutions. These two complex crystals grew in the same space group, P2(1)2(1)2(1), and diffracted to 1.6 A resolution. A second crystal form of WO2 Fab was grown in the presence of the sparingly soluble Abeta(1-42) in PEG 550 MME. This second form belonged to space group P2(1) and diffracted to 1.9 A resolution.