-ll- various organisms utilized by man for food is a major consideration when one realizes the low level of activity now considered to be the maximum per- missible amount for humans for some of the isotopes in question. 3. Why use mixed fission products in preference to known isotopes? Work with Known isotopes in the fission product series has been done and other studies should continue, but very little work has been carried out with mixed fission products. Studies leading to evaluation of possible contamination of continental sites in the advent of war or in case of accident should involve mixed fission products, not single isotopes. Basic work on the biological half life and site of deposition has been done for some forms, but studies on the biological half life and selection and retention of fission products by aquatic forms have not been done. It is known that the presence of even trace amounts of some elements may increase or hinder