  We discuss the hysteretic behavior of the field-induced variations of the
microwave surface resistance in superconductors in the critical state.
Measurements have been performed in a bulk sample of Nb and a powdered one at
different values of the temperature. We discuss a model, based on the Coffey
and Clem theory, in which we take into account the flux distribution inside the
sample, due to the critical state. The experimental results are justified
quantitatively in the framework of our model. We show that by fitting the
experimental data it is possible to determine the value of the critical current
density and its field dependence.
