Abstract Two crystal forms of Gram − bacteria TEM β-lactamase have been obtained. The tetragonal form has a very large unit cell and diffracts to 3·0 Å resolution. Orthorhombic crystals, grown using ammonium sulfate and a small amount of acetone as precipitating agents, belong to space group P2 12 12 1 with cell parameters a = 43·1 A ̊ , b = 64·4 A ̊ , c = 91·2 a ̊ and diffract to 1·7 Å resolution. A seeding procedure has been designed that ensures reproducibility of the crystal properties. Molecular replacement, using a model reconstructed from the C α co-ordinates from Staphylococcus aureus PC1 β-lactamase, gives a solution that satisfies crystal packing constraints.