Neutrino mass and baryon-number nonconservation in superstring models

We propose new mechanisms for understanding neutrino masses in superstring models that contain ${\mathrm{E}}_{6}$-singlet zero-mass fields after compactification. We show that the low-energy gauge group of these models can be phenomenologically acceptable. We then comment on \ensuremath{\Delta}B=1 and \ensuremath{\Delta}B=2 baryon-number-violating processes in these models.