  We report detailed Knight shift measurements of the two indium sites in the
heavy fermion compound CeIrIn$_5$ as a function of temperature and field
orientation. We find that the Knight shift anomaly is orientation-dependent,
with a crossover temperature $T^*$ that varies by 50% as the field is rotated
from (001) to (100). This result suggests that the hybridization between the Ce
4f states and the itinerant conduction electrons is anisotropic, a result that
reflects its collective origin, and may lead to anisotropic Kondo liquid
behavior and unconventional superconductivity.
