Oxygen does not quench the luminescence of either free Tb or of Tb bound to dipicolinate. However, sensitized Tb luminescence in complexes of that ion with elastase, thermolysin, and alpha-amylase is quenched by oxygen at rates that far exceed that with which the intrinsic fluorescence of the proteins is quenched. We infer that this more rapid quenching of Tb luminescence indicates a major role for energy transfer from tryptophan moieties in a triplet excited state.