  The asymptotical behavior of physical quantities, like the order parameter,
the replicon and longitudinal masses, is studied around the zero-field spin
glass transition point when a small external magnetic field is applied. An
effective field theory to model this asymptotics contains a small perturbation
in its Lagrangian which breaks the zero-field symmetry. A first order
renormalization group supplemented by perturbational results provides the
scaling functions. The perturbative zero of the scaling function for the
replicon mass defines a generic Almeida-Thouless surface stemming from the
zero-field fixed point.
