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Isotopes formed in the bombardment of {sub 33}As{sup 75} with 190 Mev deuterons range in atomic number up to 24 (or more) mass units lighter than As{sup 75}. Identification of these isotopes was based on chemical behavior and half-life determination. Relative yields have been calculated and show that 80% of the observed reactions produce isotopes within 8 mass units of As{sup 75}. Three new isotopes have been observed: 9.5 d. Se{sup 72} (K), 44 m, Se{sup 71} ({beta}{sup +}), and 52 m, As{sup 71} ({beta}{sup +}).

For a number of investigations, including fundamental studies of radiation effects on living tissues and therapeutic utilization of radioisotopes, it is valuable to have methods for the selective localization of radioisotopes in certain tissues. Finely dispersed anhydrous chromic phosphate has been found useful by Jones, Wrobel, and Lyons in selectively irradiating the liver and spleen with p{sup 32} beta particles. The present studies, reported in this and the following communication, are concerned with methods for controlled selective localization of colloids (incorporating radioisotopes) in the liver, spleen, or bone marrow, and with an analysis of some of the factors involved in the phenomenon of localization.

Several workers have shown that certain colloidally dispered materials are removed from the blood stream by the liver and spleen. Jones, Wrobel, and Lyons have utilized suspensions of anhydrous chromic phosphate for the selective irradiation of the liver and spleen with p{sup 32} beta particles. Gersh demonstrated that colloidal calcium phosphate is taken up by the liver and spleen. He stressed the failure of bone marrow phagocytes to take up this colloid in rats and dogs (though he referred to possible uptake in the marrow of rabbits under special conditions), and commented on the relative 'refractoriness' in general of the bono marrow as compared with liver and spleen with respect to the uptake of colloidal dyes from the blood stream. Some histological data indicate that 'Thorotrast' (a colloidal thorium dioxide preparation) is deposited in the bone marrow as well as in the liver and spleen, but no quantitative data as to the relative distribution are available. In the preceding communication the methods for the preparation of colloids incorporating radioisotopes of yttrium, columbium, and zirconium were given. The present studies are concerned with the localization of such colloids primarily in the bone marrow or primarily in the spleen and liver, with …