Fab fragments of anti-apamin monoclonal antibodies have been purified to homogeneity and crystallized. The crystals belong to the monoclinic space group P2 1 with cell dimensions a = 99.0 +/- 0.3 Å, b = 137.1 +/- 0.4 Å, c = 76.0 +/- 0.2 Å and β = 92.9 +/- 0.9 o . They most likely contain four molecules in the asymmetric unit (V m = 2.39 Å 3 /Da). The possibility of the existence of non-crystallographic symmetry is discussed.