Single crystals of bovine P2 myelin protein have been grown in polyethylene glycol 4000 by the hanging-drop vapor diffusion method. Crystals belonging to space group P2(1)2(1)2(1) with cell dimensions a = 91.8 A, b = 99.5 A, c = 56.5 A (1 A = 0.1 nm). The diffraction pattern extends to better than 2.3 A resolution.