Thymus and activation-regulated chemokine (TARC) is a CC chemokine that is most highly expressed in the thymus. TARC interacts primarily with the CCR4 receptor and to a lesser extent with the CCR8 receptor. Three different crystal forms of synthetically prepared TARC were grown in triclinic, hexagonal and tetragonal systems. The X-ray data for the triclinic crystals (unit-cell parameters a = 56.46, b = 76.48, c = 88.37 A, alpha = 85.8, beta = 72.8, gamma = 70.0 degrees ) extend to 1.85 A on a conventional radiation source. The hexagonal crystals diffracted to 2.2 A at a synchrotron-radiation source and belong to either space group P6(1)22 or P6(5)22, with unit-cell parameters a = 61.8, c = 315. A. The tetragonal crystals diffracted to about 5 A at a synchrotron-radiation source and had approximate unit-cell parameters a = b = 47.7, c = 58.2 A.