  In Gravity Unified Theories all operators that are consistent with the local
gauge and discrete symmetries are expected to arise in the effective low-energy
theory. Given the absence of multiplets like 126 of SO(10) in string models,
and assuming that B-L is violated spontaneously to generate light neutrino
masses via a seesaw mechanism, it is observed that string theory solutions
generically face the problem of producing an excessive $\nu_L-{\tilde H}$
mixing mass at the GUT scale, which is some nineteen orders of magnitude larger
than the experimental bound of 1 MeV. The suppression of $\nu_L-{\tilde H}$
mixing, like proton longevity, thus provides one of the most severe constraints
on the validity of any string theory solution. We examine this problem in a
class of superstring derived models. We find a family of solutions within this
class for which the symmetries of the models and an allowed pattern of VEVs,
surprisingly, succeed in adequately suppressing the neutrino-Higgsino mixing
terms. At the same time they produce the terms required to generate small
neutrino masses via a seesaw mechanism.
