B lifetimes from lattice simulations

We evaluate the matrix elements of the four-quark operators which contribut e to the lifetimes of B-mesons and the Lambda(b)-baryon. We find that the s pectator effects are indeed larger than naive expectations based purely on power counting even if they do not appear to be sufficiently large to fully account for the discrepancy between the O(1/m(b)(2)) theoretical predictio n and experimental measurement of the ratio of lifetimes tau(Lambda(b))/tau (B).