Large R-parity violating couplings and grand unification

We consider the possibility that R-parity violating interactions of particles from second and third matter generations have large (up to 1) coupling constants, \Lambda. Such couplings have a number of phenomenological consequences: renormalization of b-\tau mass ratio, generation of \nu_\tau mass in MeV region, etc. In Grand Unified models, where B- and L-violating couplings appear simultaneously, the proton decay can be forbidden in virtue of hierarchical flavour structure of \Lambda. However, due to Cabibbo-Kobayashi-Maskawa mixing, this decay is induced already in one-loop. Present experimental data give the upper bound \Lambda \ltap 3 \cdot 10^{-9} (or |\lambda' \lambda''| \ltap 5\cdot 10^{-17}, on the product of the L- and B-violating coupling constants, in more general context). The bounds can be avoided, if there is an asymmetry between the L- and B-violating couplings of usual matter fields. In the SU(5) model the asymmetry can be related to the doublet-triplet splitting