New experiments on a well known compound, a dodecanuclear manganese cluster of formula [Mn12O12(CH3COO)16(H2O)4, are providing very exciting results. Recently this cluster was shown to have a huge magnetic anisotropy of the Ising type which, associated to the large spin of the ground state, S=10, is responsible of an unusually slow relaxation of the magnetization. This, on its turn, is the origin of hysteretic behavior at low temperature. We have also looked at the relaxation properties in dilute frozen solutions of the cluster and we have found that the slow relaxation and hysteretic behavior persist, confirming that we are dealing with a molecular phenomenon. The large spin ground state associated with the king type anisotropy have also been evidenced in a different type of clusters of formula [Mn10(biphen)4O4Br124-recently synthesized, in which four manganese(III) and six manganese(II) are present and the spin of the ground state is 12.