  We present a simple protocol to purify bipartite entanglement in spin-1/2
particles by utilizing only natural spin-spin interactions, i.e. those that can
commonly be realized in realistic physical systems, and S_z-measurements on
single spins. Even the standard isotropic Heisenberg interaction is shown to be
sufficient to purify mixed state entanglement if there are at least three pairs
of spins. This approach could be useful for quantum information processing in
solid-state-based systems.
