  The Schwinger proper-time method is an effective calculation method,
explicitly gauge invariant and nonperturbative. We make use of this method to
investigate the radiatively induced Lorentz and CPT-violating effects in
quantum electrodynamics when an axial vector interaction term is introduced in
the fermionic sector. The induced Lorentz and CPT-violating Chern-Simons term
coincides with the one obtained using a covariant derivative expansion but
differs from the result usually obtained in other regularization schemes. A
possible ambiguity in the approach is also discussed.
