The physical properties of radioactive phosphorus (P32) make it suitable for studying the effect of beta particle radiation on the skin. Radioactive phosphorus emits beta particles only and disintegrates at a daily rate of 4.8 per cent, thus losing one half of its initial activity in 14.3 days. The beta particles of radioactive phosphorus have a continuous energy spectrum ranging from 0 electron volts (e.v.) to 1.69 × 106 e.v. The average energy of the radioactive phosphorus particles is approximately 7.0 × 105 e. v. The greatest abundance of the particles occurs at the average energy. Approximately 600 cm. is the maximum distance a beta particle of radioactive phosphorus can travel in dry air at 0° C. temperature and 760 mm. Hg pressure; in water or tissue the maximum distance is approximately 8 mm. (1–4). Technic The radioactive phosphorus used in the study to be reported here was an aqueous solution of disodium hydrogen phosphate containing 15 mg. of the salt per cubic centimeter of water. The radioactivity of the solution was measured at the Radiation Laboratory of the University of California in Berkeley and determined in microcuries per cubic centimeter. The figures reported here refer to activity measurements determined by the method and standards used at that laboratory. Discrepancies were found in the activity of radioactive phosphorus when measurements were made at different centers. Thus, when a solution is used, the activity of which has been determined at the Washington University laboratories in St. Louis, it is necessary to apply a correction factor of + 1.1, while for activities determined at the Massachusetts Institute of Technology a factor of −2.3 must be employed at the present time to obtain comparable figures (5). Early in these studies the need became apparent for a technic by which the exposures and reactions could be exactly reproduced. After prolonged trial of numerous vehicles, such as absorbent cotton, lanolin, vaseline, gum acacia, etc., it was found that ordinary thin blotting paper is a most suitable and simple vehicle for the radioactive phosphorus solution. Blotting paper of known dimensions soaked in measured amounts of Na2HPO4 solution and dried can be applied easily in contact with any part of the skin, and exposures can be reproduced exactly. Disks of blotting paper 2.5 cm. in diameter (4.9 sq. cm. in area) and 0.4 mm, in thickness (21 mg /cm2) were used in these experiments. The disks were applied over normal healthy skin on the flexor surfaces of the arms and forearms. Twelve subjects were studied in this series, and from three to five disks were applied on each subject, approximately 3 cm. apart. Three were applied on the forearm and two on the arm. Each blotting paper disk was held in place, in contact with the skin, by zinc oxide type adhesive tape an inch in width, which entirely covered the disk. Three subjects had exposures on both arms and forearms, 9 subjects on one arm and forearm only.