Any Lorentz‐invariant local field theory must obey theCPTtheorem. That is to say, the theory must be invariant under thecombinedoperations of charge conjugation, parity inversion and time reversal, even though we know that these symmetries, taken one at a time, can be violated in elementary‐particle processes. It follows that particles and their antiparticles must have identical masses and equal magnetic moments (of opposite sign).