  We analyse the problem of distillation of entanglement of mixed states in
higher dimensional compound systems. Employing the positive maps method [M.
Horodecki et al., Phys. Lett. A 223 1 (1996)] we introduce and analyse a
criterion of separability which relates the structures of the total density
matrix and its reductions. We show that any state violating the criterion can
be distilled by suitable generalization of the two-qubit protocol which
distills any inseparable two-qubit state. Conversely, all the states which can
be distilled by such a protocol must violate the criterion. The proof involves
construction of the family of states which are invariant under transformation
$\varrho\to U\otimes U^*\varrho U^\dagger\otimes U^{*\dagger}$ where $U$ is a
unitary transformation and star denotes complex conjugation. The states are
related to the depolarizing channel generalized to non-binary case.
