  The heavy fermion superconductor UPt$_3$ is thought to have odd-parity, a
state for which the temperature dependence of the spin susceptibility is an
important signature. In order to address conflicting reports from two different
experiments, the NMR Knight shift and measurements of the anisotropy of the
upper critical field, we have measured the bulk susceptibility in a high
quality single crystal using polarized-neutron diffraction. A temperature
independent susceptibility was observed for $H||a$ through the transitions
between the normal state and the superconducting A-, B- and C-phases,
consistent with odd-parity, spin-triplet superconductivity.
