  The density matrix of a spin s is fixed uniquely if the probabilities to
obtain the value s upon measuring n.S are known for 4s(s+1) appropriately
chosen directions n in space. These numbers are just the expectation values of
the density operator in coherent spin states, and they can be determined in an
experiment carried out with a Stern-Gerlach apparatus. Furthermore, the
experimental data can be inverted providing thus a parametrization of the
statistical operator by 4s(s+1) positive parameters.
