R-Parity Violation and Peccei-Quinn Symmetry in GUTS
  We address the question whether it is possible in GUTs to obtain R-parity violation with a large ${\Delta L{/}\Delta B}$ hierarchy of strengths so that the proton is stable while phenomenologically interesting $L$-violation is present. We consider versions of SU(5) with a built-in Peccei-Quinn symmetry spontaneously broken at an intermediate scale. The P-Q symmetry and the field content guarantee a large suppression of the effective $B$-violating terms by a factor $\Lambda^3_{PQ}/M_PM^2_X $ while the effective $L$-violating terms stay large. 
K. Tamvakis (CERN)