THE calculation of tissue dosage from gold radon seeds in terms of a reproducible standard or unit is essential not only that we may know the dose administered to the tumor itself, but also to the surrounding healthy tissue. This is particularly important when external radiation is applied in addition to interstitial implants. Pending the establishment of a standard unit, we have in the past endeavored to relate the intensity from seeds (1), from intracavitary radiation (2), and from surface application of radium tubes, to the degree of reaction of the skin (erythema) produced by applying a 100-milligram tube of radium for 450 milligram-hours at a distance of one centimeter. These conditions are easily reproduced. The specifications of this tube are as follows: Length over all 21.5 millimeter Outside diameter 4 millimeters Wall thickness 1 millimeter of platinum Radiating length 19.5 millimeters When applying this tube to the skin, a spacer of 8 millimeters of wax was used, making the distance from the ce...