. . . clinical observations . . . indicate that there is a very definite difference between the skin and biological reactions in relation to the physically measured r dose. The main influencing factors requiring consideration are: (a) Quality in relation to depth; (b) size of area irradiated; (c) dose intensity, and (d) rhythm.1 Most roentgen ray treatments have been given to areas of 5 cm. diameter or larger. The effect of the size of the irradiated tissue on the dosage has not been fully realized till of late. This paper will deal with the "size of the small area irradiated" as an influencing factor on the biologic reaction of the tissue treated. The other influencing factors mentioned will be kept constant. Considerable work in this regard has already been done.2 Why does the size of the area treated have such an influence? The answer to this is bound up with