  We have studied numerically the quantum-classical crossover in the
escape-rate for an uniaxial spin system with an arbitrarily directed field.
Using the simple quantum transition-state theory, we have obtained the boundary
separating the first- and the second-order crossover and the escape-rate in the
presence of the transverse and longitudinal field. The results apply to the
molecular nanomagnet, ${\rm Mn_{12}}$.
