  We determine thermal entanglement in mean field clusters of $N$ spin one-half
particles interacting via the anisotropic Heisenberg interaction, with and
without external magnetic field. For the $xxx$ cluster in the absence of
magnetic field we prove that only the N=2 ferromagnetic cluster shows
entanglement. An external magnetic field $B$ can only entangle $xxx$
anti-ferromagnetic clusters in certain regions of the $B-T$ plane. On the other
hand, the $xxz$ clusters of size $N>2$ are entangled only when the interaction
is ferromagnetic. Detailed dependence of the entanglement on various parameters
is investigated in each case.
