Mixed-State Entanglement and Distillation: Is there a “Bound” Entanglement in Nature?

It is shown that if a mixed state can be distilled to the singlet form it must violate partial transposition criterion A. Peres, Phys. Rev. Lett. 76, 1413 (1996). It implies that there are two qualitatively different types of entanglement: ``free'' entanglement which is distillable, and ``bound'' entanglement which cannot be brought to the singlet form useful for quantum communication purposes. A possible physical meaning of the result is discussed.