  We study the effect of integrating out the heavy fields in a supersymmetric
GUT which does not contain small mass parameters in the limit of exact
supersymmetry. The trilinear ($A$) and bilinear ($B$) coefficients of the
supersymmetry soft-breaking terms of the low-energy effective theory are
related in a simple and model-independent way to those of the underlying
theory. {}From these relations, we obtain the bound $|B|\ge 2$, which, together
with the requirements of stability of the potential and electroweak symmetry
breaking, imposes severe constraints on the space of allowed supersymmetric
parameters. In models based on supergravity with a flat K\"ahler metric, we
obtain $B=2$, instead of the relation $B=A-1$ usually used in phenomenological
applications. The low-energy theory contains also a supersymmetric mass term
$\mu$ for the two Higgs doublets, which is of the order of the
supersymmetry-breaking scale.
