In the effective field theory of quantum gravitation coupled to N_s scalars, N_f fermions, and N_V vectors, tree unitarity is violated at an energy squared of E_{CM}^2 = 20(G_N N)^{-1}, where N = {2/3}N_s + N_f + 4N_V and G_N is Newton's constant. This is related to radiative corrections proportional to G_N N E^2 (where E is the typical energy), due to loops of such particles. New physics must enter before E_{CM}=5.6 x 10^{18} GeV in the standard model, and 4.6 x 10^{18} GeV in the minimal supersymmetric standard model.