  We present the technical tools needed to compute any one-loop amplitude
involving external spacetime fermions in a four-dimensional heterotic string
model a` la Kawai-Lewellen-Tye. As an example, we compute the one-loop
three-point amplitude with one "photon" and two external massive fermions
("electrons"). As a check of our computation, we verify that the one-loop
contribution to the Anomalous Magnetic Moment vanishes if the model has
spacetime supersymmetry, as required by the supersymmetric sum rules.
