Crystals of enzyme IIB of the cellobiose-specific phosphotransferase system have been obtained from 15% polyethylene glycol 4000 using both streak-seeding and macroseeding techniques at 4 degrees. Crystals were grown with the hanging drop method of vapour diffusion. Addition of 2-propanol and benzamidine/HCl proved essential to obtain single crystals suitable for X-ray analysis. The crystals diffract to 1.8 A resolution and have the monoclinic space group P2(1), with cell dimensions a = 53.6 A, b = 31.7 A, c = 60.0 A and beta = 101.7 degrees. From a self-rotation function it seems likely that there are two molecules in the asymmetric unit related by a non-crystallographic 2-fold axis.