Glycosylated interleukin-2 (glyIL-2) has been crystallized in two crystal forms, and unglycosylated interleukin-2 (uIL-2) has been crystallized in three forms. The glycosylated form of the human recombinant IL-2 has been crystallized from 1.9 M ammonium sulfate, pH 6.5 to 7.0 in the hexagonal space group P6(2)22 or its enantiomorph. The crystals diffract to 2.8 A and contain two or three molecules per asymmetric unit. A second crystal form grows from 1.4 to 1.5 M ammonium sulfate in 0.2 M ammonium acetate, pH 5.0-5.5, as polycrystalline rosettes which are not suitable for even a preliminary crystallographic analysis. The uIL-2 crystallizes from 1.0 to 1.7 M ammonium sulfate, 0.2 M ammonium acetate, pH 4.5-5.6 in the monoclinic space group P2(1), and less frequently in the orthorhombic space group P2(1)2(1)2(1) from 2.5 M ammonium sulfate, pH 4.5 to 5.7. Cross-seeding uIL-2 with seeds from hexagonal crystals of glyIL-2 promotes nucleation of trigonal crystals of unglycosylated IL-2. These trigonal crystals belong to the space group P3(1)21 or its enantiomorph, with similar cell dimensions to the glyIL-2 hexagonal crystals.