Experimental and clinical studies evaluating the usefulness of radioactive microspheres in treatment of tumors have been in progress since 1960. Ceramic or plastic in composition, these spheres have been found to have many uses in industry. As carriers of radioactive isotopes they have several features favorable to medical application.1 Almost any cation can be incorporated as the microspheres act like exchange resins. The bond is then made permanent by firing in a kiln. Very high specific activities can be achieved (up to 10 curies Y90/gm). The spheres can be accurately segregated into groups of uniform size varying from 15μ to 500μ. Finally, the spheres themselves are biologically inert and resist leaching even in strong acids and/or alkalis. In experiments previously reported by LaFave and associates2 it was shown that the administration of Y90 microspheres into the rabbit mesenteric vein along with VX2 carcinoma cells prevents