Let G be a locally compact group, and let G(d) denote the same group G with the discrete topology. There are various C*-algebras associated to G and G(d). We are concerned with the question of when these C*-algebras are isomorphic. This is intimately related to amenability. The results can be reformulated in terms of Fourier and Fourier-Stieltjes algebras and of weak containment properties of unitary representations.