Fab fragments of mouse anti-(4-hydroxy-3-nitrophenyl)acetate (NP) antibodies N1G9 and 3B44 have been crystallized in 6 crystalline forms. Antibody N1G9 is one of the primary immune response antibodies. 3B44 is from the secondary response and exhibits a 10-fold higher affinity than N1G9. Form I crystals of the N1G9 Fab were crystallized using ammonium sulfate as a precipitant, and diffract X-rays to 2.4 Å resolution. Crystal forms II, III, IV and V were obtained from polyethylene glycol solutions. They belong to a monoclinic space group and contain two Fab molecules per asymmetric unit. The 3B44 Fab, which has 15 amino-acid mutations from the N1G9 Fab, was crystallized only from a polyethylene glycol solution. The Fab elbow angle in the 3B44 structure from a 2.9 Å resolution study on the NP-soaked crystal is 206°, larger by 9° than that of the N1G9 Fab. Different molecular contacts in the crystal lattices are supposedly originating from this difference in the elbow angles as well as from some of the mutations, and are affecting the crystallization properties of these Fabs.