Interest has recently increased in the methods of delivering high doses of radioactive antitumor agents into localized anatomic areas. Technics with some success include the intracavitary (1, 2) and the intralymphatic (3–7). For this purpose Burger (7) chose radiochromic phosphate (CrP32O4). The phosphorus 32 has a half life of fourteen and a half days and a maximum tissue penetration of 8 mm and, in addition, is a pure beta emitter. The radiochromic phosphate delivers an energy of 885.000 rep (roentgen equivalent physical)∕mc∕ml, or ten times more than a colloidal radiogold solution of the same activity. In view of the importance of possible radiation damage to healthy tissues and the lack of information on this subject in the scientific literature, we decided to study the distribution of particles of different sizes of colloidal chromic phosphate (P32) in tissues susceptible to radiation damage such as bone with its marrow and the gonads. Liver was employed as a reference tissue because, according to Schubert (8), its uptake differs with particle size: higher for the large size and lower for the small size. Material and Methods The colloids were prepared by a modification of the author's technic (9) which provided two different colloidal preparations: one a true colloidal solution (Type 1) with a particle size up to 0.3 micron; the other a suspension of particles condensed into aggregates (Type 2) with a particle size of 0.6 to 2.0 micra (Fig. 1). Before use, both preparations were analyzed by electrophoresis to assure a soluble phosphate-free condition. The same dose (25 μc) of each type of colloidal preparation was injected into adult white mice divided into three groups. In the first group the site of injection was the tail vein (intravenous), in the second, the site was the pleural cavity (intracavitary), and in the third, the site was the abdominal muscles (intramuscular). Ten animals from each group were sacrificed at different intervals: one, seven, and fifteen days after injection. To count the total radioactivity, the whole bodies of 5 animals in each group were ashed at 600 to 700° C., and the ash was dissolved in concentrated hydrochloric acid and diluted to 1 N. The organs of the other 5 animals were dissected, and the livers, testes, ovaries, and femurs were pooled and ashed under the same conditions as previously described. Finally, aliquots of each mineralized fraction were neutralized with 1 N sodium hydroxide, dried under an infrared lamp, and counted with a thin-window Geiger-Müller counter. To make the self-absorption corrections, a self-absorption curve was obtained by measuring samples containing different weights of mineralized material with the same added P32 activity.