The Klyachko inequality is an inequality for the probabiities of the values
of five observables of a spin-1 particle, which is satisfied by any
noncontextual assignment of values to this set of observables, but is violated
by the probabilities defined by a certain quantum state. We describe an
experiment between two entangled spin-1 particles to test contextuality via a
related inequality. We point out that a test of contextuality by measurements
on a single particle to confirm the Klyachko inequality requires an assumption
of non-disturbance by the measuring instrument, which is avoided in the
two-particle experiment.
