The translocation of a polymer driven by an external bias field of strength epsilon across a model membrane has been investigated using Monte Carlo methods. It is found that below a characteristic strength of the field epsilon*, the membrane is practically impenetrable. In the high-field limit, epsilon > epsilon*, the permeability mu = tau 0/tau, where tau(0) and tau are the translocation times of the polymer without and in the presence of the membrane, respectively, is independent of epsilon. At low fields the permeability decreases according to mu similar to exp(-epsilon*/epsilon). The translocation mechanism can be understood as a Kramers process describing the escape of a Brownian particle over potential barriers.