Single crystals have been grown of Cd,Zn metallothionein isoform II from rat liver. The space group is P4(1)2(1)2 (P4(3)2(1)2) with unit cell dimensions a = b = 31.0 A and c = 120.0 A, and one molecule in the crystallographic asymmetric unit. The crystals are square bipyramids elongated on the tetragonal c-axis and are grown by repetitive seeding. The crystals are suitable for high resolution structure analysis. Assays of dissolved crystals show that the crystals have the same Cd and Zn content and amino acid composition as the native, as-isolated protein.