We report preliminary results of our study of matrix elements of baryon number violating operators which appear in the low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The calculation is performed on a $32^{3}times80$ lattice at $beta=6.1$ using Wilson fermions in the quenched approximation. Our calculation is independent of details of (SUSY-)GUT models and covers all interesting decay modes.