Because it binds soluble forms of proteins with disease-associated polyglutamine expansions, the antibody 3B5H10 is a powerful tool for studying polyglutamine-related diseases. Crystals of the 3B5H10 Fab (47 kDa) were obtained by vapor diffusion at room temperature from PEG 3350. However, the initial crystals gave highly anisotropic diffraction patterns. After optimization of the crystallization conditions and cryoprotectants, a nearly isotropic diffraction pattern at 2.6 A resolution was achieved for crystals with unit-cell parameters a = 133.26, b = 79.52, c = 41.49 A and space group P2(1)2(1)2. Dehydrated crystals diffracted isotropically to 1.9 A with unit-cell parameters a = 123.65, b = 78.25, c = 42.26 A, beta = 90.3 degrees and space group P2(1).