  Supersymmetric Grand Unified Theories have a rich spectrum of particles
barely heavier than the intermediate vector bosons. As their non-supersymmetric
counterparts, they lead to many relations among low energy observables. But the
precise form of the predictions is modified by the extended spectrum. If the
masses of these new particles are comparable to $M_Z$, the standard computation
of their effect becomes inaccurate. We present a detail discussion of the
correct procedure, and carry out the relevant computations to one loop order.
The procedure we propose has the advantage over other existing methods that
two-loop running of gauge couplings can be incorporated readily and
consistently. Attention is paid to the special treatment that the top and Higgs
particles must receive. The size of the effect is explored for a range of
parameters in the minimal supersymmetric $SU(5)$ grand-unified theory with
radiative breaking. It is found that the naive (leading-log) computation can be
fairly inaccurate.
