Four steps in the development of any particular observable change in a cell resulting from exposure to radiation are: (1) absorption of radiant energy, (2) primary radiochemical reactions, (3) chemical reactions, and (4) observable lesion. Steps (2) and (3) constitute the latent period. The term "protection" is proposed to describe a diminution of the observable lesion resulting from a treatment or procedure which is operative before the beginning of irradiation, and the term "restoration" when this is achieved by treating the cell in some way after the beginning of irradiation.