Crystals of recombinant human granulocyte-macrophage colony-stimulating factor have been grown from solutions of polyethylene glycol 8000. The crystals are orthorhombic, space group P2(1)2(1)2(1); the axes are a = 45.5(1), b = 58.7(1) and c = 127.3(1) A. The crystals are stable to x-rays for at least 3 days and diffract beyond 2.8-A resolution. Although the molecule exists as a monomer in solution, it crystallizes with two or three molecules in the asymmetric unit.