  We evaluate the SUSY and top threshold effects in the context of the MSSM and
the string derived model based on SU(4)$\times$SU(2)$_L\times$SU(2)$_R$. In
both cases we run the two loop RGEs and determine the lower bounds of the
supersymmetric particle masses, dictated by the experimentally accepted regions
of the values of the low energy parameters.
