The mechanisms of inactivation of deoxyribonuclease by direct action of radiation have been studied. By testing kinetic properties of the enzyme and its tryptophan content, it seems likely that when the enzyme molecule absorbs energy from radiation, the energy is transferred to the ‘active site’ of the enzyme. This results in the degradation of one tryptophan residue, which, in turn, causes a loss of catalytic function of the enzyme. The differences between indirect and direct actions are also discussed.