From the experimental upper bound on the decay mu --> e-gamma the mixing of a heavy neutral lepton (m > 45 GeV) with nu(e) and nu(mu) can be constrained to be \U(Ne)*U(N-mu)\2 < 7 x 10(-6). In turn this implies a contribution to K(L) --> mu-e of no more than 2 x 10(-15) in the branching ratio.