  In supersymmetric extensions of the standard model there is no basic
difference between Higgs and matter fields, which leads to the well known
problem of potentially large baryon and lepton number violating interactions.
Although these unwanted couplings can be forbidden by continuous or discrete
global symmetries, a theoretical guiding principle for their choice is missing.
We examine this problem for a class of vacua of the heterotic string
compactified on an orbifold. As expected, in general there is no difference
between Higgs and matter. However, certain vacua happen to possess unbroken
matter parity and discrete $R$-symmetries which single out Higgs fields in the
low energy effective field theory. We present a method how to identify maximal
vacua in which the perturbative contribution to the $\mu$-term and the
expectation value of the superpotential vanish. Two vacua are studied in
detail, one with two pairs of Higgs doublets and one with partial gauge-Higgs
unification.
