The success of Lawrence and his coworkers (1, 2) in the treatment of leukemia with radioactive phosphorus (P32) suggests that this isotope may also be of value in the treatment of certain malignant neoplasms. In order to obtain knowledge as to the immediate deposition of p32 in cancer tissue, small amounts of this isotope, about 0.5 microcurie, in the form of a 1.5 to 3.0 per cent solution of Na2HP04, have been administered at varying times before operation to patients with the following operable neoplasms: carcinoma of the breast, osteogenic sarcoma, lymphosarcoma. Small portions (about 2 grams) of the different tissues in the surgically excised specimens were weighed to 0.5 milligram, ashed at 500° C., and their radioactivity determined by direct comparison with accurately measured fractions of the original solution given to the patient. The final values were expressed in terms of microcuries per kilogram of tissue and were corrected for decay to the date of administration of the P32. Specimens with activities of 0.1 microcurie or less were measured with a Geiger counter, and those with activities of more than 0.1 microcurie were measured with a Lauritsen electroscope. Specimens which had a large amount of ash were suitably subdivided to prevent error due to absorption of the beta rays by the ash. It is to be expected that the concentration of the isotope in tissues after such “tracer doses” is representative of concentrations to be found after the administration of therapeutic amounts of the isotope, provided the difference between the amounts of phosphate administered in each instance is not too large. In order that proper account could be taken of the variable weights of patients and the differences in the amounts of isotope administered, the concentration of p32 in tissue, expressed in microcuries of p32 per kilogram of tissue, was divided by the microcuries of p32 administered per kilogram of body weight. The resultant figure was termed the “differential absorption ratio” 3 (D.A.R.). When this is done a ratio is established between the amount of phosphorus actually absorbed by that tissue and the amount it would have contained if the isotope had been equally distributed throughout the whole organism. This ratio shows at once whether a neoplastic tissue absorbs an amount of the isotope sufficiently above the average for the entire body to make it a potentially useful method of therapy. If, for example, a neoplastic tissue has a differential absorption ratio of 1, it will receive no more radiation from the radioactive phosphorus than will the whole body. On the other hand, if the ratio were 6, it would receive 6 times as much radiation as the average body tissue. In the first instance, p32 would be expected to be of little value; in the second instance, it could be of significant therapeutic effect.