Large R-parity violating couplings and grand unification

We consider the possibility that R-parity violating interactions of particles which do not involve the first generation have large (up to 1) coupling constants, Lambda. Such couplings, if they exist, could have a number of phenomenological consequences: renormalization of the b - tau mass ratio, generation of nu(tau) mass in the MeV region, etc. In grand unified models, where B- and L-violating couplings appear simultaneously, proton decay can be forbidden in virtue of the hierarchical flavor structure of Lambda. However, due to Cabibbo-Kobayashi-Maskawa mixing this decay is induced already at one loop. Present experimental data give the upper bound Lambda less than or similar to 10(-8) (or \lambda'lambda ''\ less than or similar to 7 x 10(-16), products of certain L- and B-violating coupling constants, in a more general context). The bound can be avoided if there is an asymmetry between the L- and B-violating couplings of the usual matter fields. In the SU(5) model the asymmetry can be related to the doublet-triplet splitting.