Large crystals of probable amylase/protease inhibitor-B have been grown at room temperature from ammonium sulfate solution. The crystals grow within five days to dimensions of 0.6 mm x 0.6 mm x 0.6 mm. They diffract to at least 1.7 A upon exposure to synchrotron X-rays. The crystals belong to the space group P4(1)2(1)2 (or P4(3)2(1)2) with unit cell dimensions of a = 38.02 A and c = 98.98 A. The presence of one molecule per asymmetric unit gives the unit cell volume per protein mass (Vm) of 1.99 A3/Da and the solvent fraction of 38.2% by volume. X-ray data have been collected to 2.0 A Bragg spacing from native crystals.