S100A12, a member of the calgranulin family, isolated from human blood, has been crystallized by vapour diffusion in the presence of Ca(2+). Crystals belong to the space group R3 with unit-cell dimensions a = b = 99.6 c = 64.2 A. There are two monomers per asymmetric unit, with a solvent content of 57.9%. The crystals diffract to at least 2.2 A resolution and complete X-ray data have been collected to 2.5 A on a conventional laboratory source.