  We study the existence of a spin-glass phase in a field using Monte Carlo
simulations performed along a nontrivial path in the field--temperature plane
that must cross any putative de Almeida-Thouless instability line. The method
is first tested on the Ising spin glass on a Bethe lattice where the
instability line separating the spin glass from the paramagnetic state is also
computed analytically. While the instability line is reproduced by our
simulations on the mean-field Bethe lattice, no such instability line can be
found numerically for the short-range three-dimensional model.
