A secreted form of the beta-chain of a T-cell receptor specific for a hemagglutinin peptide of influenza virus in the context of the major histocompatibility complex class II I-Ed molecule has been crystallized in a form suitable for X-ray diffraction analysis. The crystals are tetragonal, space group P4(1)2(1)2 (or P4(3)2(1)2), with cell dimensions a = b = 71.4 A, c = 312.9 A, and diffract to beyond 3.5 A resolution. The beta-chain appears to behave as a stable homodimer in solution.