RECENT discussions of the mechanism of high-temperature superconductivity have focused on the symmetry of the superconducting order parameter (the wavefunction of the superconducting state) as a means of constraining the nature of the interaction that forms the Cooper pairs(1,2). There have been many attempts to measure the symmetry of this parameter in the high-temperature superconductor YBa2CU3O7-delta (refs 3-10), but they have not yielded consistent results, Here we show that half-integer flux quantization in superconducting rings with three grain-boundary Josephson junctions, which formed the basis of an earlier, less conclusive symmetry test(3), can be used to place firm constraints on the pairing symmetry, For the case of YBa2Cu3O7-delta, we find that the sign of the order parameter is clearly direction dependent (consistent with a d(x2-y2) symmetry) and that the contribution from any out-of-phase isotropic component is less than about 3%.