particles of the various isotopes. Thus it tekes 20,000 ~ 30,0C0 rep from Ss’? (ave. energy 0.05 mev.) to produce a reaction, while it takes only 1,500 ~ 2,C00 rep of sr”? or r? (av. evergy 0.3, 0.7 mev.) to produce the same reaction. The degree of skin damage therefore is dependent on the absorbed dose at a certain critical depth in the skin. Moritz and Henriques found that the dose at 0.09 millimeters depth of te pig skin (estimated to be the epidermal thickness) was constant within several hundred rep to produce transepidermal injury. Wilhelmy has also noted that it takes roughly the same dose of electrons and | soft X rays at the level of the subpapillary layer to produce erythema. On this basis, Parker has advocated the use of beta~detecting instruments with chamber walls corresponding in milligrams per square centimeter to the thick- ness of the relatively inert epidermal layer. Thus in expressing dkin dosage, it is probably more informative to use the depth dose at a level corresponding to the basal cell layer of the epidermis. Table 1 also indicates the species difference in skin sensitivity to beta radiation, Rabbits and sheep required larger doses than mice to produce the same effect with roughly the same energy beta. Porcine skin, which is re- putedly more like human skin than other animals, apparently is more sensitive than the rabbit or sheep skin. Some of these differences, aside from species differences, may be due to variation in thickness of the epidermis of different species and differences in techniques used. Table 2 shows beta dosage data from some iuman experiments and accidents found to produce various effects on the skin. These data mst be interpreted with great caution due to differences in experimental techniques and dosimetry. The authors have taken the liberty of interpreting the severity of the skin reactions given by these investigators in degrees. A first degree reaction im- plies erythema and/or dry desquamation; a second degree, transepidermal necrosi